I also have the same problem this happened after I flashed the latest firmware T295XXU4BUD4. Now I cant flash android 9 bootloader and when I tried downgrading it won't allow me to. Can anyone pls help me with this I will send the files if required.Tell us what firmware came on your tablet. Also what firmware you downloaded as "stock", and what firmware you downloaded as Android 9.
When you say "failed", do you mean you flash something and the tablet no longer boots? Or boots, but Magisk not installed? Or no sign that flashing changes anything?