All: The issue causing corruption for the JFFS partition on the RT-AX86U has been found, and a fix provided by Asus has been tested with success.
However, the following models were also affected by the same issue: RT-AX68U, GT-AXE11000, and possibly the RT-AC68U_V4 (unconfirmed on that one).
If you have any of these three models, and you are running either 386.7 Alpha 1 or Beta 1, I strongly recommend that you take the following steps:
1) Make a backup of your settings and JFFS partition (same page where you can make a backup of your settings)
2) When Beta 2 comes out, after flashing it, reformat your JFFS partition
3) After the reboot with reformat, restore your JFFS backup, and reboot again.
If the device fails to boot with Beta 2, then do a factory default reset. After booting and configuring your basic settings (wifi/password/etc...) then format JFFS, reboot, then re-import both settings and JFFS, and reboot.
The issue is related to the layout of the flash partitions which had changed. The fix will restore the original partition layout, however there is a strong chance that this can lead to JFFS corruption if you were running with the incorrect partition layout, hence the need to use a jffs backup.