Hi,
I might remember this incorrectly, but wasn't the case that whenever you typed space in say, the name of a group, Houdini would automatically replace it with and underscore ‘_’ character? It's still the case when naming a node, but in a group name, space chars are no longer substituted and it results in an invalid name.
I've looked in the change log and the first 5 pages doesn't seem to address this. I might've missed it, but I haven't updated H since long before .500 so it's easier to just ask here.

Group field is just a string field, there's no way it can know you are typing a literal group name and not an expression that may contain valid spaces.