I've noticed this issue with a few games now, but the current one is
Chosen, so I'll reference that in particular. But it seems like URM isn't good at identifying parent and child variables, and it's certainly unable to freeze any child variables.
For instance, in Chosen, there's a variable for the time, but it's under clock.time. However, even searching for "clock" turns up nothing, not even as the parent. In other games, even if the parent shows up in the search, and you can expand it to view all the child elements, while you can save the variable to your variable list, you can't watch or freeze that variable in particular.
I don't know if this is simply a ren'py an engine limitation, but hopefully it's fixable.