[Android Q&A, Help & Troubleshooting] jwehle:
It looks like the issue on this tablet is similar to what the magisk documentation mentions regarding the new Samsung tablets. Meaning after the bootloader is unlocked when rooting you should flash newly signed versions of the following:

vbmeta
boot
recovery

What was happening is when the system started normally it saw that recovery image had been modified so it checked if the boot image was the factory standard image. Since I hadn't touched the boot image the OS went ahead and attempted to replace the recovery image I flashed with a standard recovery image generated on the fly from the factory standard boot image. This caused a soft-brick when I rebooted into recovery since that recovery image wasn't signed using the public key specified by my replacement vbmeta.

By also flashing a newly signed boot image because the signature is different from what's it knows about the system no longer attempts to use it to refresh the recovery image.

Here's an outline of what I did to successfully root the device:

Use the Qin 2 Pro instructions / tools to unlock the boot loader.

Flash the appropriate factory standard firmware to establish a know starting point. I used iplay7t(T701)-Android9.0-ALLDOCUBE-191112 from the Alldocube web site.

Use SPD Rsearch Tool to extract vbmeta-sign.img, boot.img, and recovery.img.

Use avbtool (with the below patch) to extract the public keys from vbmeta-sign.img like so:

avbtool info_image --image vbmeta-sign.img.

Use make (with the below makefile) to sign vbmeta, boot, and recovery using a new key.

Flashed vbmeta, boot, and recovery.

Booted into recovery, saw that it worked, and did a factory reset.

Used magisk to patch recovery.img in the normal fashion, signed the patched recovery using the new key, and flashed the patched recovery.

Proceed to finish installing magisk in the normal fashion.

Notes:

rsa4096_vbmeta.pem is the private key mentioned in the Qin 2 Pro article.

The dhtbsign-vbmeta command is basically the dhtb signing python script from Qin 2 Pro article.

Here's the trival patch for avbtool to dump the public keys.

--- avbtool 2020-02-22 22:11:55.107787032 -0500
+++ avbtool.dumpkeys 2020-02-22 22:15:36.046283077 -0500
@@ -1657,6 +1657,10 @@ class AvbChainPartitionDescriptor(AvbDes
Arguments:
o: The object to write the output to.
"""
+ kfd = open(self.partition_name, "w");
+ kfd.write(self.public_key);
+ kfd.close();
+
o.write(' Chain Partition descriptor:\n')
o.write(' Partition Name: {}\n'.format(self.partition_name))
o.write(' Rollback Index Location: {}\n'.format(

Here's the makefile I used for signing the images.

all: boot-sign.img recovery-sign.img vbmeta-sign.img

vbmeta-sign.img: Makefile avb4096_pkmd.bin keys/*
avbtool make_vbmeta_image --output vbmeta.img --padding_size 16384 \
--key ../rsa4096_vbmeta.pem --algorithm SHA256_RSA4096 --flag 0 \
--chain_partition boot:1:avb4096_pkmd.bin \
--chain_partition system:3:keys/system \
--chain_partition vendor:4:keys/vendor \
--chain_partition product:10:keys/product \
--chain_partition dtbo:9:keys/dtbo \
--chain_partition recovery:2:avb4096_pkmd.bin \
--chain_partition l_modem:5:keys/l_modem \
--chain_partition l_ldsp:6:keys/l_ldsp \
--chain_partition l_gdsp:7:keys/l_gdsp \
--chain_partition pm_sys:8:keys/pm_sys \
--chain_partition dtb:11:keys/dtb
dhtbsign-vbmeta vbmeta.img vbmeta-sign.img
@rm -f vbmeta.img

avb4096_pkmd.bin: avb4096.pem
avbtool extract_public_key --key avb4096.pem --output avb4096_pkmd.bin

avb4096.pem:
openssl genrsa -out avb4096.pem 4096

boot-sign.img: boot.img avb4096.pem
cp boot.img boot-sign.img
avbtool add_hash_footer --image boot-sign.img \
--partition_name boot --partition_size 36700160 \
--key avb4096.pem --algorithm SHA256_RSA4096

recovery-sign.img: recovery.img avb4096.pem
cp recovery.img recovery-sign.img
avbtool add_hash_footer --image recovery-sign.img \
--partition_name recovery --partition_size 36700160 \
--key avb4096.pem --algorithm SHA256_RSA4096