Yeah, the whole story is kinda shady. And all those female megaera teachers regularly abusing schoolchildren. Of course, "they suspected that he was up to no good".
I had the same stamp from the Ho Chi Minh City consulate. It was just before Covid, so I didn't try to get more tourist visas since then. But I doubt it would have any real implications.
Do they have legal grounds to refuse a tourist visa if you meet all the requirements listed on their website? There is no mention that you should have no more than a certain number of tourist visas in the past.
Did you see a previous thread by OP? It's the second time they haven't let him out. The first time he didn't have an overstay. They just said he had too many covid visas.