It's not too surprising, considering the way the rules are written at the ATF. There's basically zero logical thought that goes into pistol vs rifle vs felony:
https://www.reddit.com/r/Firearms/comments/a4gnr3/makes_perf...
(Sorry for the reddit link, it's a common image but that was the first url I found from a quick search that had it up front and center).
The ATF has been in court (and lost) quite a bit [1] over this.
[0] there's a nice picture and writeup here of a pistol brace being setup https://www.thetruthaboutguns.com/gear-review-sig-sb15-pisto...
[1] a brief rundown of the 2023-2025 legal rulings https://www.fflguard.com/atf-pistol-brace-rule/
US gun laws are bizarre.