[Nexus 6P Q&A, Help & Troubleshooting] samlingx:
Edit 5: Gonna go ahead and declared this solved, the problem hasn't come back since I factory reset and reflashed MHC19I (not Q) from scratch. No more issues with wifi not autoconnecting or taking excessively long to connect, no more battery drain (got nearly 48 hours this weekend including a good amount of time spent in a poor/oversaturated service area at the Grand Prix), etc. Probably going to wait until the May update before flashing any OTAs.

Edit 4: Two days later and still seems to be working fine. I'm seeing some other issues on the April update (significant battery drain, slow wifi connect, etc.), so this morning I tried rolling back first to January (wouldn't get LTE) and now to March (fine so far). With the exception of keeping my media, I'm setting this up completely new, i.e. redownloading everything from the Play Store, to avoid any weirdness from restoring backups.

Edit 3: A factory reset done via TWRP (which retained my media) seems to have solved it for now. At any rate it's been acting normal longer than anything I've done before. This includes restoring my apps + their data via TitaniumBackup (but not system backups), reflashing SuperSU and otherwise getting my phone back to where it started. I've tested things that were known to cause it before like having wifi active but leaving my signal area and having it revert to cell data. The only issue I've noticed now is that wifi no longer seems to autoconnect when it's in range of my router. Not a huge deal but kind of annoying, if anyone has a fix for this I'd appreciate that too.

Edit 2: This issue is NOT resolved. The problem came back after working for a little while. Once the LTE fails the wifi will never reconnect on its own. It seems like everything still works except loading images from anything (reddit, instagram, etc.) and loading bookmarks. I can make google searches, download apps, go into reddit comments, go to instagram profiles I've never been to before, and so on. Is my device defective?

Edit: It appears that going through the motions of flashing the latest update (save for recovery.img and formatting userdata) has fixed the issue. This includes reverting to the stock kernel and flashing the vendor.img, which I hadn't done before. Wifi can be flipped off and on and will reconnect and there are no "!"s anywhere in sight. Strange considering I essentially went through the whole process minus the stock kernel and vendor.img, just out of order, with no effect.

Can anyone offer insight as to what piece of that process might have fixed this? It's not clear to me which pieces of the update are responsible for wifi and cellular connectivity besides the radio and kernel. Leaving the original post below in case anyone has anything to add or is going through this same issue. I'll update this post if the issue returns.

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As of just this afternoon I'm suddenly getting a strange new issue that I can reproduce pretty consistently. I first noticed that my wifi signal was claiming to be connected but with no internet, and showed a little exclamation point next to it. I disabled wifi, and noticed the exclamation point jumped over to LTE. In both cases, I can still essentially use the connection. I disabled cellular data with wifi connected and could still browse the web, and likewise if I disabled wifi and the exclamation point was back on the LTE symbol. However, I notice some apps like Instagram will refresh posts but no longer load images, and reddit will load posts but not follow any links or load any images either.

Things I've tried:

-Flashed the 2.50 radio (down from 3.61)

-Flashed the MHC19Q system.img and cache.img

-Installed ElementalX kernel (at first it looked like it fixed the LTE issues, but really it just made the "!" go away; still seeing the same issues when I connect to wifi and then disconnect)

-Deleted and readded the nxtgenphone APN (I'm on AT&T)

-Tried to get into *#*#4636*#*#; I get "Connection problem or invalid MMI code."

-Cleared cache/dalvik in recovery (hey, you never know)

-Rebooted my router to make sure it's not an issue there; all other wireless devices on my router are fine

-Removed and reseated the SIM card

In all cases I can still send and receive texts and certain apps will continue to function about halfway, such as Instagram mentioned above. Switching airplane mode on and off doesn't help. Furthermore, once the wifi connection first gets the "!", it will no longer automatically reconnect as it doesn't find a working internet connection.

After an initial reboot it will appear to be fine; wifi connects automatically, no "!"s anywhere in sight, and everything seems to work. As soon as I disconnect from wifi, this issue starts. As mentioned, it will no longer reconnect to wifi and cellular data doesn't seem to work.

Anyone have any thoughts or suggestions as to what the issue may be? This started out of the blue this afternoon, I haven't made any changes to my phone since flashing the April image. My bootloader is unlocked and I'm rooted, and save for the ElementalX experiment (going to go back to the stock kernel) everything else is stock.

Any help is appreciated. If you need any more info, let me know.