The contents of first.js seems to include AMD modules from all themes currently installed, I would have expected only the current theme modules to be loaded.

Previous discussion:
https://moodle.org/mod/forum/discuss.php?d=325073

Unlike other modules, there's no mechanism to have themes installed but "hidden" by clicking an eye icon, so to avoid this happening it seems like you need to physically remove the theme from the disk. Doing this to bootstrapbase seems unwise since it's part of core Moodle, and it wasn't previously something that needed to be done for other installed themes.

If a future version of Moodle ships with both a Bootstrap 4 default theme, and Bootstrapbase for backwards compatability, the two versions of the Bootstrap js would both be loaded by themes that use either as a parent theme.