My first rule of thumb is that any medication can cause anything.

I have seen the most obscure and unusual adverse reactions to commonly used medications. This is not surprising if you think that one small mutation in one protein can lead to a host of changes in the entire organism. Pharmacological agents never work on one system. All we know from large scale studies is that in the vast majority of the population this medication has a significant beneficial effect, without significant unwanted reactions. But, this does not mean that it will be so in the entire population. A mutation in one protein, that would have no effect in life, can lead to significant and unexpected changes once a pharmacological agent is added (and there are numerous known examples for that, and probably numerous more that have not yet been described).

Practically, If you clearly see a relation between your symptoms and the mestinon your are taking (eg-dose-wise and time-wise) then even if it has never been described before, this is your response to this medication.

If those side effects are worse then the illness itself, then clearly it does not make sense to keep on taking this treatment. take into account that mestinon is no more and no less than symptomatic treatment, so if you have worse symptoms with it, than without it, why take it?

I stopped taking mestinon, because it made my illness much less stable. It sometimes worked like Popeye's spinach, sometimes did nothing, and sometimes made it much worse. As it is the most bothersome aspect of my illness is the extreme and unexpected fluctuations, so even though with mestinon I had times when I had a near-normal strength, it was not worth the price of other times in which I had to be in bed for a few days connected to my respirator.

none of my neurologists understood why this is so (or even tried to understand it). most of them just told me it is "impossible". I have possible explanations, but it doesn't really matter. we don't always know the cause of what is happening, yet we do know for sure that it is happening.
Many times in science and medicine, the explanations are found years after the first observation.