I can’t answer your question. The “__trashed” suffix is added by WP when you create a new post that has the same slug as a post that is in the trash so that the new post can use that slug. I’ve seen this myself.
The only thing that I can think of that might cause this, but I’m not sure. I have not tested.
1) Import or Sync a field group
2) Trash the field group
3) Import or Sync the same field groups
4) Trash this field group
5) Restore the original field group
WP should be removing the “__trashed” suffix from these posts I think, but it’s hard to follow.
You might want to submit a ticket here to let the developer know about it https://www.advancedcustomfields.com/contact/
