Any rules against using capitals in the naming of fields?

I have some very old fields (circa Sugar v.6.x) that have names that start with capital letters like: VIP_status_c

I just now noticed that in v9.0.3 Professional and v.10.0.3 Professional when I add them to a layout using Studio, the layout is saved, the timestamp on my file system updated, but the field is not added to the layout.

I tried this in List View and in Search with three different fields.

If I manually edit the view and do a QRR it accepts them, no problem.

Is this a known issue? Should I be planning to rename my fields to make them compliant with some rules I'm not aware of?

thanks.
Francesca