mx and ipv6

Until recently I was publishing a very simple 'v=spf1 +mx -all'. Then
I received an email from someone who said that his system showed a
fail as my MTA sent the mail to his using IPv6, and that the mx only
causes an 'A' DNS lookup not an 'AAAA' and therefore the '+mx' did not
match the IPv6 of my MTA. Whereas an mx query using dig returns both
the A and AAAA data in the 'Additional section'. So should the SPF
'mx' not match both IPv4 and IPv6 addresses for the MTA?