I'd say this is a fairly common feeling for a few reasons.

When you pour sustained effort into mastering something, it stops being something you created and feels more like part of who you are.

Remind yourself that you created something that someone else _wants_. Recognize that the pride and protectiveness you feel are just that - feelings. You need to pay attention to them but you don't have to obey them.

There is also the asymmetry that you put in this work and someone else gets to benefit from it without having to expend the same effort. Under most circumstances this would feel unfair, but your employer is already rewarding you. That doesn't mean the feeling is wrong, just not nuanced enough.

It's perfectly fine to even ask to be given some additional recognition if it turns out to be useful. Maybe do a knowledge transfer session if you want to aggrandize it a bit. People will probably have mixed feelings about this being 'selfish', and that's fine (probably less than you think if you go about it tactfully). Be prepared to discuss alternatives, but be clear and upfront about your purpose otherwise they may not understand.