[EDIT 03/17/2022] This has been resolved (see explanation below, this post)
[EDIT 04/02/2022] This has not been resolved! See post #366
Very astute Niwi! I have no idea how long this has been happening.
Some initial tests:
1) if the player Wild Shapes their Druid, the player's AC stat is incorrect and the DM's stat is shown correctly. Further, Fantasy Grounds resolves an attack roll correctly.
2) if the DM Wild Shapes the player's Druid the player's AC stat is shown correctly but the DM's stat is incorrect. And Fantasy Grounds does not resolve attacks against the Druid correctly.
Fantasy Grounds is using the AC of the DM's copy of the character sheet to resolve the attacks against the Wild Shaped Druid.
So, if the player Wild Shapes their Druid, even though the AC stat is wrong in their copy of the character sheet, Fantasy Grounds correctly resolves attack rolls against the Wild Shaped Druid.
Restated: The player should Wild Shape their Druid and things will be okay, though their character sheet is wrong.
I am extremely busy with non D&D issues until the end of October. I'll try to get this fixed before then if I can squeeze in some time now and again to work on it.
Note that I did not test player on player attacks, (e.g. collateral damage from a spell).
[EDIT] Feb 17, 2022
This appears to have resolved itself, on its own. Today, when I started looking into it, I could no longer reproduce the error. I suspect changes were made to OOB messaging (in the system builtin functionality) at some time in the past which caused the error. And then that functionality was again modified more recently to my favor. Should the odd behavior reported by Niwi be observed again please let me know. Hopefully I will be able to address issues in a more timely manner in the future.