For myself, also with MTHFR, I have only ever heard of a problem with metformin in relation to B1, thiamine. To compensate, I supplement with benfotiamine, a derivative of thiamine that does not use the same uptake route as metformin.

I later found out that metformin permanently alters your ability to absorb vitamin B12.

If you ever get a real source for this (the actual study and not just a hearsay claim) I'd love to know about it. But honestly, I don't think you'll ever find one: as others have pointed out, that just not how B12 or its uptake works. While the resulting damage (i.e. neurological) from severe or long-term B12 deficiency may be permanent, not being able to utilize it is a whole different story, for example pernicious anemia.

Whoever said these things got a number of different biological systems, conditions, and beliefs confused. It's like a medical word salad, frankly.

If you're worried, don't be... but do look into supplementing with B1 or benfotiamine: it will not only mitigate whatever thiamine losses metformin cause, but it is very helpful in Type 2 in its own right.

Hope this helps.