I just tested this and don't have the same issue.

Is this on local play? I haven't had this issue on when I play or online.

if you dont deactivate the person playing as a mega eevee then try to have another mega eevee the game just freezes becoming unable to close the game without restarting your computer

I believe it is the same unless I miscalculated. Have you noticed anything?

is this weaker or stronger then base evee's mega eevee evolution