I tried with https://file-examples.com/storage/fe42a6a13e6a80e779c3cf0/2017/04/file_example_MP4_480_1_5MG.mp4 and https://www.youtube.com/watch?v=tAGnKpE4NCI in Opera 134.0.5954.56 64-bit on Windows 11. Hammered left and right while the seek bar is focused to rewind and fast forward. Tried for quite a while and can't seem to reproduce it.
I also tried focusing a blank spot on the page at https://example.com/ and hitting left and right a lot to see if I could trigger it that way. Couldn't reproduce it there either.
You can try updating Opera to the newest build to see if you can still reproduce.
You can test with the "Hardware Media Key Handling" flag at the URL opera://flags/#hardware-media-key-handling disabled to see if there's any difference. Sometimes that flag (on by default) causes issues.
But, I'm guess the issue will still be there for you where it's just something that I can't personally reproduce.