Is that with specific extensions rather than all extensions? I do sell multiple extensions outside of marketplace (via itch.io) and haven't had people report being unable to import them, whereas in that context that would have meant that people would have virtually no way to import (without editing project files by hand, which is horrible)
I don't know if the case is .yymp from outside of marketplace and how your users do it (perhaps not using drag n dropping or importing from marketplace library? another way?). Because on a version of GMS2 (i don't remember the exact ver. number) it was working, but after some updates released, it doesn't work anymore until now (drag n dropping). Since that event (not working) i always download assets directly inside IDE library now.
And as of today i can confirm again, if i drag n drop assets (.yymp) from YoYo marketplace via web browser, and save the file to hardisk, and then import it by drag n dropping, GMS2 IDE not able to recognize it at all. Perhaps the percentage of Repro is not 100% happening for others? idk but as far as i tested it multiple times it doesn't work 100% of time on my end.
Btw, last time YoYo confirmed the bug report, the problem was with 2.0 IDE importing the asset, not the asset/marketplace, which strangely it works if we just import it from inside library, but not by drag n dropping it (the same .yymp file isn't it?).
I don't know whether it's only for specific assets or for all assets, but in my case and others reported it not working so far it doesn't work with different assets.
Related to your assets, wouldn't you like to test it again today (drag n dropping it to GMS2 workspace) on the latest GMS2 IDE and confirm it (despite the assumption that your asset users not reporting error so it must work)? If it works so it is not 100% of repro and only for specific assets because yours are working