If suddenly some scenes render with a watermark, but others don’t, that probably means someone saved those scenes with Skip License Check enabled.
A quick way to check this is with the Arnold log.
If you set the verbosity level to at least Warnings + Info, you would see this:
00:00:00 787MB WARNING | [rlm] skip_license_check enabled
if Skip License Check is enabled.
If you’re using MtoA and rendering Maya scenes, one way to avoid this problem is to render with the -ai:slc off flag (slc = skip license check).
In the very unlikely case that Skip License Check isn’t the problem, the set the verbosity level to Debug and check the log again.