Despite failing to win any of AV-TEST's best antivirus awards for home users, Microsoft Defender was considered as one of the best in the market by the anti-malware assessment firm. And although AV-Comparatives wasn't impressed to that degree, Defender in general received praise from it too.
And despite all the applaud, Microsoft's anti-virus solution is far from flawless as just recently, it was found that Defender for Endpoint was flagging its own Office updates as malware, and now reports of Defender SmartScreen causing video playback issues on the latest Beta build 22581 are coming in.
Several users on the Windows 11 subreddit flighting the build 22581 are reporting video playback delays. The mention of MKV is there but other video container formats may also be affected. ISOs too apparently are facing this delayed execution problem. The issue isn't listed among the known issues for the build which means it probably went undetected during testing.
A simple workaround for this problem was figured out by Reddit user kristijan1001 who realized that disabling the "Check apps and files" option under SmartScreen settings was able to resolve the issue. But before proceeding to apply this workaround, make sure the file type is actually trustworthy.