trilogy6202 VanadiumConfig version 13 will include a fix for this issue. Currently in Beta channel.
GrapheneOS account on Discord: "please try updating to the new VanadiumConfig version 13 in Alpha if you're having the autofill provider reset issue
this may be an issue in your autofill provider app causing the compatibility detection to unset it
we're going to try turning off the compatibility detection which is what the latest VanadiumConfig does".
It seems to have fixed it for me.