What Is This Called?

Didn't know where else to post this.

What is the name of the psychological paradox that effects at least some people where they have a subject/hobby/etc. they really enjoy, but when it becomes obligatory or they're paid to do it, the subject can lose its appeal?

Specifically the occurrence of someone really enjoying x, tasks related to x are now an obligation, person now prefers to do other tasks related to x that aren't productive and don't satisfy obligations?

I know this is not unique as I've heard of other people mention this before.

Arw there any materials, discussions, etc. that cover this?

Lastly, and sage pieces of advice or information for someone that has this issue.

Thanks