Change Subtotal labels in Matrix visual

Hello, After the May release that changed some of the format locations and functionality, I'm having an issue with a Matrix visual.

I have turned off the stepped layout option and I’m trying to get only a few of the rows to show subtotals and want to name each subtotal section. I’ve been able to change the subtotal like the red below before, but for some reason I can’t get it to change on this one.