I was able to install mame... However I did so using the custom feature.
I added the same name. Arcade (MAME) and it uses the Arcade theme. However.... I dont think theres any other theme media... game default theme for instance.
Ive burned enough time on this... hopefully its helpful.
I can't help with the technical aspects of your post, but there is a default game theme for dreamcast. If it didn't auto-install, you can download it here (just make sure HQ is running):
HyperSpin Theme - HyperTheme
I would like to add that it's not just the most recent version of MAME. It's all versions of MAME. The most recent version is not a contributing factor to the issue.
all good guys, with new HyperHQ update, now i can update my existing mame database, so i guess they fixed it, now it removed the extra in the dabatase, problem resolved!