Sharpening 4K footage makes it blurrier

I’m having a strange problem with Final Cut Pro X. I have imported 4K footage. I have used this method for two years and up until now it has worked fine. Sometimes I like to zoom into the footage using the transform tool. Let’s say 200%. Because I’m filming in 4K this will still look great even when viewed on YouTube on a phone.

But I’m noticing to strange things that never happened before:

Firstly, the 4K footage although as sharp as possible at 100%, looks degraded and blurry at 200%.

Secondly, when I sharpen that footage, which usually helps a lot, it immediately gets twice as blurry as before. It’s almost as if the sharpen effect is actually a blur affect.

I am working directly on synchronised footage dragged from the file viewer into the timeline.

Any ideas?