Screengrabs are not malicious in and of themselves. So, there is no need for anti-virus or anti-malware to be suspicious of that behaviour. Every OS has some sort of screenshot program. But unusual screengrabbing can increase the “score” for suspicion.
What anti-virus or anti-malware will do is look at the behaviour in context and comparison to all the other behaviour it has, or if it is a known bad binary, it will detect it.
I used to run a commercial, non-malicious PC spying package that took a screenshot every time someone hit the enter key or clicked the mouse. (This was a very specific use case for a specific threat on a single machine, and the machine had a poster informing users of this). I needed to whitelist the package’s processes in the anti-malware software we used because it was the combination of behaviours (screengrab hooked in the background to the keyboard and mouse actions in all processes and not just the screengrab program) that was suspicious.