Thx for the suggestion, I forgot about doing that. However, VirusTotal shows a 0 / 59 for any threats.

I used MKVToolNix to checkout the video file. Inside the MKV container appears to be: an H264 video file, an AAC audio file, 3x VobSubs, chaptering info, and 'global tags'. If I uncheck the 'Global Tags' entry and save the rest an a new MKV, the video opens in Windows 10 without any warning message from Windows Security.

I don't see anything in the properties data for 'global tags' that looks suspicious, or even has any entries at all (such as for timestamps, video properties, color information, color mastering meta information, etc. I don't know WHY having a 'global tags' "thing" in the MKV is causing the security warning.

Unless there's a way to have Windows 'ignore' the 'global tags' part of an MKV, I guess I will just re-multiplex the videos with that part removed.

(Sorry it took so long to reply, I didn't have my lemmy password saved to my online password manager and had to wait until I got back to my media PC to update the posts)

Thanks for the replies

This is what VLC shows in media information for the file

I thought I embedded the warning message in the first post, trying again here