One more check: the only thing that differs between OK and FAIL tests - is SSL? Or there are different MongoDB versions? MongoDB is not friendly to community, and I think there could be changes in versions which will break compatibility, so versions of servers is important (at least to know it they differs or same).

If I were you, first thing to do is to search call stack sub by sub to find where this error comes from, placing debugging prints everywhere. Then I would try to file an issue (https://github.com/oliwer/mango/issues). If filing an issue will not resolve the problem, try to reach Sebastian Riedel himself on mojo IRC (https://web.libera.chat/#mojo) and ask him what to do. I've done it myself when stuck in Mojolicious. He have sharp tongue, but what could you do? And then, if nothing helps, you can always return to call stack one more time, and try to find the cause yourself. Who knows, where this adventure will lead you to?