Contenttype Groups/Tabs don't work when Group name starts with a number

I notice that starting a `contenttypes.yaml` group name with a number stops the backend tab from working.
So `group: Three topimages`  works OK but `group: 3 topimages`  makes that tab non-clickable. 

Possibly a JS thing, it could also be related with CSS, where #ids names  are not allowed to start with a number.

Bolt 5.1.10, I believe