SMF offers the option to set a theme by board, and not force it on anyone who has a specific theme selected.
For this to work, the user needs to have the "Forum or Board Default" theme option selected in their profile.
For an admin user, this does not work, and so an admin is unable to see the board specific themes without forcing them for everyone.
Longer explanation of the same in Selecting "Forum or Board Default" theme as Admin seems to be not working (https://www.simplemachines.org/community/index.php?topic=558084.0)
I can replicate the same bug on 2.0.15, it only seemingly resets for admins if the board default setting is already set.
https://github.com/SimpleMachines/SMF2.1/issues/4525