If you make or modify any mask using the Brush tool, then you leave ACR and then open it again, the brush tool's work is not retained. Other masking tools like gradients and person selection seem ok on their own, however if you try to modify ANY mask with the Brush, it disregards the mods made with the brush when saving and relaunching ACR.
e.g., use the Masking brush tool to brighten a face; close ACR. Return to ACR to make other adjustments, and realize the face you masked and it's brighter settings are not there, so you have to repeat your work and immediately export it in order to keep the results. However, any return to ACR requires starting over...
This was with Canon .CR3 files, but it also failed with a .NEF used for testing purposes.
I've tested on 2 mac computers with the same results. On my older mac, I tested before I updated from ACR 16.2.1 to 16.3 and the other mac was already on 16.3 when I noticed the problem.
(Newer) M1 MBPro Mac's Adobe apps are current:
ACR 16.3
MacOS 13.6.6
Bridge 14.1
PS 25.9.0
(Older) Late 2013 MBPro has:
ACR 16.2.1 initially (and working correctly), and then I specifically updated ACR to 16.3 (brush masking tool fails) to prove my theory.
MacOS 11.7.10
Bridge 13.0.5
P 25.4