I believe I read on this forum that this is a bug in only converted apps. I believe you have to delete the button and then recreate it to fix it.
Unfortunately, I believe that you are correct. I was just checking to see if it was just me. (I tried to search the forums before posting).
I maintain an unofficial list of Known Bug List here in the forum. I can't find any mention of such a bug in my database. I suggest using Report an Issue (see tab at top of this screen) to report this. If it's confirmed by a FileMaker Inc. "TS" person, I'll then add it to the list.