when MAXTARGETS= exists in raw 005, mIRC knows it can use the #chan,#chan,#chan target format instead of sending multiple commands to the server, but it strikes me as odd that it doesn't look at the actual value of MAXTARGETS and split up the messages accordingly
for example, MAXTARGETS=3 and you're on 5 channels #a,#b,#c,#d,#e. /amsg will send:
PRIVMSG #a,#b,#c,#d,#e
when it should send:
PRIVMSG #a,#b,#c
PRIVMSG #d,#e
i haven't tested it for other similar commands like /ame, /qme, /qmsg (maybe even /omsg, /onotice etc.) but i imagine they also have this problem. tested on 6.21 of course