@cambreco yes, I believe you. I had the same for a long time today. As you say, the original amount wasn't editable to issue to a partial refund. Once I worked with QBO support on the phone with screenshare, then it changed for me.

Before: this screen below had grey Amount/Comment field (locked), noneditable

After support call help: these 2 fields turned blue and it's editable now and I can partial refund.

I've sent through a couple partial refunds now.

It's mysteriously buggy. All I can assume is the support personnel resolved it. I didn't do anything differently before/after.