Upon further thought, I don’t.
The decay rate prior to the flare would be due to neutrinos leaving the sun before the flare.
Yes, that would be the reason (release of neutrinos or some other fast particle before the flare).
I shouldn’t really have mentioned neutrinos though. Yes they are fast enough, and because we can’t really detect them it makes sense to suspect their involvement.
But neutrinos really do have a lousy collision cross-section with most matter. Unless Manganese turns out to be the periodic tables champion neutrino collector, and we can also trace a role in neutrinos and radioactive decay.