> you can use
> \startMPinitializations
> \stopMPinitializations
> as these are expanded each graphic run


Thank you! This works fine with minimal example (in which the
behaviour in question appears).
But in specific context in which this problem affects me, this
solution breaks many things:

There are many *MPinclusions with as many MPinstances (so variables
from one instance don't contaminate another).
(here's that place:
https://github.com/jemmybutton/byrne-euclid/blob/master/preamble.tex#L30)

If this is indeed a bug and it's going to be fixed one day, I'll go
with some less invasive hack to overcome it,
but if this behaviour is correct, I'll try to rewrite everything accordingly.

Or I got it wrong and *MPinitializations can substitute for
*MPinclusions in this case?

Sergey