A logic issue was addressed with improved checks. This issue is fixed in Safari 27.2, iOS 26.7.2 and iPadOS 26.7.2, iOS 27.2 and iPadOS 27.2, macOS Golden Gate 27.2. A website may be able to use a media format that should be restricted in Lockdown Mode.
https://support.apple.com/en-us/149527
https://support.apple.com/en-us/149521
https://support.apple.com/en-us/149230