Why do they often write the single word “hifu” with one kanji and one katakana character?

The full kanji version 皮膚 (skin) has a second kanji that is no longer in the official list of 2000 or so that are learnt at school, so it is often written 皮フ outside doctor's etc. Why they don't just have done with it and switch to all katakana or hiragana I'm not sure.

I think I've seen a few more similiar examples, but I can't remember what they were and this is by far the most noticeable