[Google Pixel 6] badabing2003:
Thanks @V0latyle

I did some reading.

The successful bit needs to be set to true, otherwise boot process will not boot to it.

You can see the flow and the required bits in Google's official document here

@Jaitsu

Put your device in bootloader mode and hit the Get Info button in PF

Check the following values and see what you get.

(bootloader) slot-count:2
(bootloader) slot-fastboot-ok:a:yes
(bootloader) slot-fastboot-ok:b:yes
(bootloader) slot-retry-count:a:2
(bootloader) slot-retry-count:b:2
(bootloader) slot-successful:a:yes
(bootloader) slot-successful:b:yes
(bootloader) slot-suffixes:_a,_b
(bootloader) slot-unbootable:a:no
(bootloader) slot-unbootable:b:no

Thankfully @capntrips has a tool specifically for this DevinfoPatcher

Interestingly though, perhaps @capntrips can explain what might be going on here.

I put this to test:

I first launched the DevinfoPatcher and got a screen that reflected the following state (next code block).

Sorry I did not think of taking a screenshot at that point (but there was no Patch button, suggesting that everything was good)

(bootloader) slot-count:2
(bootloader) slot-fastboot-ok:a:yes
(bootloader) slot-fastboot-ok:b:yes
(bootloader) slot-retry-count:a:2
(bootloader) slot-retry-count:b:2
(bootloader) slot-successful:a:yes
(bootloader) slot-successful:b:yes
(bootloader) slot-suffixes:_a,_b
(bootloader) slot-unbootable:a:no
(bootloader) slot-unbootable:b:no

Rebooted to bootloader and dumped the properties

Which had the above.

Then I issued fastboot --set-active=b

Immediately after I read the slot info and I got this

(bootloader) slot-count:2
(bootloader) slot-fastboot-ok:a:yes
(bootloader) slot-fastboot-ok:b:no
(bootloader) slot-retry-count:a:2
(bootloader) slot-retry-count:b:3
(bootloader) slot-successful:a:yes
(bootloader) slot-successful:b:no
(bootloader) slot-suffixes:_a,_b
(bootloader) slot-unbootable:a:no
(bootloader) slot-unbootable:b:no

I was surprised because all I did was switch the slot and it marked (bootloader) slot-successful:b:no and (bootloader) slot-fastboot-ok:b:no

I had not even rebooted at this point yet

Next I rebooted back to bootloader

fastboot.exe reboot bootloader

This time it read

(bootloader) slot-count:2
(bootloader) slot-fastboot-ok:a:yes
(bootloader) slot-fastboot-ok:b:yes
(bootloader) slot-retry-count:a:2
(bootloader) slot-retry-count:b:3
(bootloader) slot-successful:a:yes
(bootloader) slot-successful:b:no
(bootloader) slot-suffixes:_a,_b
(bootloader) slot-unbootable:a:no
(bootloader) slot-unbootable:b:no

Then I tried to reboot to system slot b

Counter decremented but did not boot to system

(bootloader) slot-count:2
(bootloader) slot-fastboot-ok:a:yes
(bootloader) slot-fastboot-ok:b:yes
(bootloader) slot-retry-count:a:2
(bootloader) slot-retry-count:b:2
(bootloader) slot-successful:a:yes
(bootloader) slot-successful:b:no
(bootloader) slot-suffixes:_a,_b
(bootloader) slot-unbootable:a:no
(bootloader) slot-unbootable:b:no

Tried it one more time, and the counter decremented again.

I finally switched back to slot a and booted to system

At this point, DeviceinfoPatcher showed this.

View attachment 5779743

I hit the patch button and got this

View attachment 5779745

Reboot to bootloader

(bootloader) slot-count:2
(bootloader) slot-fastboot-ok:a:yes
(bootloader) slot-fastboot-ok:b:yes
(bootloader) slot-retry-count:a:2
(bootloader) slot-retry-count:b:1
(bootloader) slot-successful:a:yes
(bootloader) slot-successful:b:yes
(bootloader) slot-suffixes:_a,_b
(bootloader) slot-unbootable:a:no
(bootloader) slot-unbootable:b:no

fastboot --set-active=b

And it's back to

(bootloader) slot-count:2
(bootloader) slot-fastboot-ok:a:yes
(bootloader) slot-fastboot-ok:b:no
(bootloader) slot-retry-count:a:2
(bootloader) slot-retry-count:b:3
(bootloader) slot-successful:a:yes
(bootloader) slot-successful:b:no
(bootloader) slot-suffixes:_a,_b
(bootloader) slot-unbootable:a:no
(bootloader) slot-unbootable:b:no

Tried reboot multiple times till the counter reached 0 and marked the slot unbootable

(bootloader) slot-count:2
(bootloader) slot-fastboot-ok:a:yes
(bootloader) slot-fastboot-ok:b:yes
(bootloader) slot-retry-count:a:2
(bootloader) slot-retry-count:b:0
(bootloader) slot-successful:a:yes
(bootloader) slot-successful:b:no
(bootloader) slot-suffixes:_a,_b
(bootloader) slot-unbootable:a:no
(bootloader) slot-unbootable:b:yes

One more reboot, and it booted to system by switching to slot a as expected.