Dunno about FL300, but Positive Lightning is not "normal" lightning:
NWS JetStream - The Positive and Negative Side of Lightning
NWS Hastings, NE
An average bolt of positive lightning carries a current of up to 300
kA (kiloamperes) (about ten times as much current as a bolt of negative lightning), transfers a charge of up to 300
coulombs, has a potential difference up to 1
gigavolt (one billion volts), and lasts for hundreds of milliseconds, with a discharge energy of up to 300
GJ (gigajoules) (a billion joules)
Could this much energy have the effects described in the ACARS even if it wasn't a direct hit but merely in close vicinity?