I have never noticed the grid snap turning itself on, but then I rarely annotate in schedules.
I have noticed the grid lines turning themselves on in schedules, details, or when editing GDL, etc. Even sometimes when creating a new elevation.
I definitely always have the grid lines turned off and they do turn themselves on, and I know no way to stop it.
Maybe the 2 issues are related.
All I can suggest is be sure you are not accidentally pressing the keyboard shortcut, and have that toolbar handy so you can switch it off easily (or use the shortcut).
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10