First of all, compost is not soil. When filling holes before sodding or seeding you should use good topsoil, not compost. Over time that compost will decompose and sink down. Another issue is how much of the stump was left behind. If you only ground down a few inches of it, then grass will struggle in that area as it's roots won't be able to go deep, and the soil that is there will hold little water. Wood under ground rots very slowly, so don't think the stump and roots will decompose anytime soon.

Good to know about compost. A professional came and ground it way down - I forget what their standard depth is, but it's at least a foot down.

I think you're right, filling it with compost was a bad idea. Anything I can do about it? Maybe I just get a soil test and figure out was additives I need for that small area?

The main additive you need is soil. Compost is only good for about one thing and filling holes is not it. You can dig out any remaining compost and refill with soil or you can start start adding topsoil from the top as the compost 'evaporates.'