QuickTime default for M4V blocked by security warning on Mac

I have a number of personal videos in m4v format that I'm sorting through. The default app to open these files is Apple TV but I want to view them in Quicktime player. Each time I have to choose it as an alternative to open these files. So I tried to make Quicktime player the default app for m4v format. However each time I do this I then get a security message saying "Apple could not verify ... is free from malware..." and invite me to trash the file. This is complete rubbish as it happily plays the files individually if I select Open with other... Quicktime. It only happens when I try to make Quicktime the default app for this format.

The system then defaults back to Apple TV for m4v files, complete with all its ads and notifications which is what I don't want. I just want to play a video, on a straightforward video player, without being bombarded by all sorts of extraneous stuff. I understand Apple may prefer us to use its preferred app so they can do this, but please give us back some control and don't force it on us like this.

I am using an iMac 2019 with OS Sequoia 15.7.5.