That is absurd and unfortunate.
If you could reliably reproduce the problem, I'd say that showing them the problem with an "approved EVSE" they must have on-site should help you convince them to take you seriously. Another approach could be to ask them to point you to MINI's list of approved devices (which almost certainly does not exist), then press them about whether or not public charging infrastructure is "approved" or not (again, with any supporting documentation from MINI).
But at this point it's clear you're not dealing with someone willing to be reasoned with, or certainly not competent enough to diagnose/solve your problem. I think you may be better off escalating to MINI USA.