Func_breakable not sticking?

I select a few brushed all right next to each other, and hit CTRL+T. I change it to a “Func_breakable” and give it the variables I want. I hit “Apply”, but when I select something else, it turns back into a bunch of "Func_detail"s again.

Restart hammer?

I’ve had the same problem; it’s probably caused by clicking some random button on the keyboard, because pretty much everything is bound to some secret feature that no one knows about.

no, that is because you are working outside of groups.

there is a Sidebar at the right hand side of your hammer.
You probably have Selected Solids, select groups. (Solids lets you work outside of groups, when you need to change the geomtry of a solid inside a group for instance)