They worked the same in Opus 12. I think they've always worked that way, by design.
There are CASE=extupper and CASE=extlower which only affect the extension, FWIW. For something more complex, a rename script or something using evaluator code could do it (which is best depends on how/where you want to use the commands).
Now I understood, when I read the manual, I thought that "filename" referred to the name when in fact it refers to the name and the extension as a whole, sorry for my bad interpretation of English.