While that is a GIMP limitation when saving, it has no trouble opening and viewing a GIF with separate colormaps for each frame (which it opens in RGB mode if I’m not mistaken). There is a plugin that interfaces with more advanced GIF software (gifsicle) to do more advanced GIF stuff. I’m not sure how well it works; it has been years and years since I used the GIMP and I’m not about it install it on my wizened old MacBook (using X11 apps on OS X is a thing to avoid if you can).
Whereas that fly by night software package I use for GIFs is way better at optimizing for 256 colors overall, and also opens them mostly okay, but best I can tell it isn’t capable of saving with a separate palette per frame, and I know of no plugins that do this. One could always use separate software to accomplish this, though; that didn’t stop being an option that exists. (So... Adobe CS 278 - GIMP 6.)