You do realise it can be both?
The mandate loss on non tributary border was a design decision. No matter if it was wrong or not, it was designed that way and not a bug.
I think there is aw big issue in your argument. You keep arguing for mechanics to be changed due to a lack of historical accuracy but also argue against a historically accurate deterministic approach as seen in earlier PDS games.
You also argue how unfair it is that ming china has these "debuffs", when other tags who failed during the time dont get them:
I am sorry to have this completely backfire for you but all the examples you noted, with the exception of india, are set up for failure via game mechanics. Not all of them are as "on the nose" as the ming disaster (Timurids and Poland are), but they all are set up for failure due to different game systems, ranging from negative acceptance modifier for alliances due to heretic religion (screws over byzantium). Dimishing effectiveness of cav only armies (hordes) to tech and institution setup (american natives). Ming is easily not alone in the "set up for failure" category and it honestly has a much better shot than all the other noted countries. Even in the worst case scenario the disaster is only gonna result in a few loans for the player. Mings abundance of forts makes devastation meaningless, their manpower pool makes rebels meaningless and the disaster events feed back more than enough mandate. Actually so much that experienced players might want to trigger the disaster intentionally to quickly chain reforms. Even the AI handles it more often than not.
Now China has some serious issues in EU4, but those are mostly related to potential successor dynasties. And those mostly can't be solved because the whole tag system doesnt work with chinese dynasties