I don't think this is a bug. It's the same idea as SPSET resetting sprite variables to zero, it's just undocumented.
MPSTART resets MPSET values
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SquareFingersCreated:
On one system:
MPSET 0,1 MPSTART 2,"Test" PRINT MPGET(MPLOCAL,0)On the other:
MPSTART 2,"Test" PRINT MPGET(1-MPLOCAL,0)Both will print 0.
If it's undocumented, it's a bug in the documentation.
I think the documentation for MPSET is clear on that point:
Gets user-defined data from a specified terminal in a wireless communication sessionIf the session isn't started before using MPSET, undefined behavior should be expected. In this case, initializing the state of every system at the start of a MP session is reasonable.