In general if a belief is held on account of authority and is not otherwise established as a truth, then it may be true or not.
Whether it should be dismissed without further evidence depends on the nature of the proposition being natural or extraordinary as the extraordinary claims require extraordinary evidence.
The claim that one's beliefs can not be verified, if true, doesn't necessarily mean that one can't rightfully go beyond the foremost wrong view which is that the claim may or may not be true but that only if the claim can be falsified by contradiction.
If the claim can neither be falsified nor verified then one is limited to the foremost wrong view.
In another hypothetical situation, I can prove that i have been lied to (and mistreated) by my teacher. Infact, historically it has happened many times.
Is it inconsistent reasoning for me to suggest, that the credibility of that teacher doesn't diminish, just because my teacher has lied to me previously?
I am not sure what else would diminish one's credibility if not being observed to not be credible.
It does not however establish that he is not credible in a particular circumstance and it does not necessarily mean that he is particularly prone to being dishonest but if his claim may or may not be truthful then the foremost wrong view should be weighted towards it not being true.
It is like throwing a dice which may or may not be loaded. As the frequency of throws increases so does one's confidence as to whether there is a deviation from the norm, however due to the probability distribution one can never tell if one is being incredibly unlucky or is working with a loaded dice as the probability of a certain outcome never goes to zero but only gets closer to zero as in infinite series of 1/6*1/6*1/6... and one's view should thus be weighted accordingly.
It is literally impossible to throw the dice infinite amount of times and it is like if one had the power to jump infinitely far one couldn't jump to infinite distance because one wouldn't land and that isn't what jumping is, likewise when it comes to being able to teleport to any uncapped distance, one has to resolve on a finite distance because otherwise it's not an instance of teleportation.