when i researched inter-platform fast loader some upgrade to fast others stay the normal yellow version
i did have an extra mod but when i disabled it and researched it again it was the same result
i try it multiple time and every time the same result with here and there an extra loader that does not upgrade
but the there are 3 that never upgrade
the extra mod i had was filter loader and max stack loader

https://ibb.co/5hVCL0hL

Looking at the issue and how the loader upgrade is managed in the code, it means you destroyed and/or replaced the loader at some point, which breaks some relations stored in the mod data.

Does changing the yellow loader direction change the direction of the associated loader on the other platform as well ?

If you really didn't do anything (except using the other mods), can you share your mod list / your save ? Because i'm clearly not able to reproduce the issue unless I replace the loader using the editor mode.

from what you say that is indeed the problem those 2 mods add a checkbox to the loader and if you ever click it once it's a different loader
when i turn of those mods all my normal loaders are gone and if a checkbox is checkt on a dimension loader it's also gone so i needed to uncheck those if i wanted tho turn them of
unchecking doen not chance them back and wont upgrade
i fixed mine when i chose to get more loaders
it does not give you an extra loader your game goes to a state where there are more loaders
i saw the loader flicker and mij filters where gone
i tryed again and it upgraded normaly so those 2 mods are not compatibel for now
https://mods.factorio.com/mod/lane-filtered-loaders?from=search
https://mods.factorio.com/mod/loaders-make-full-stacks
here are the mods for incompatibel mention or potential fixing
thx for checking

thanks for the check / troubleshoot.

I'll have a look, and worse case, put them as incompatible