I managed to reproduce the issue on a fresh portable installation. I just made sure these options were checked:

1) mIRC Options > Other > Confirm... > Confirm when pasting 10 or more lines of text
2) mIRC Options > Display > Options > Editbox lines: Single

The next pass is to connect to an IRC network.

Then copy text longer than 10 lines. I tested with more than one text. I'll leave this text below as a test:
Code
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
bbbbbbbbbbbbbbbbbbbbbbbbbbb
cccccccccccccccccccccccccccccccccccccccc
ddddddddddddddddddddddddddddddddddddddddddd
eeeeeeeeeeeeeeeeeeeeeeeee
ffffffffffffffffffffffffffffffffffff
ggggggggggggggggggggggggggggggggggggggggggg
hhhhhhhhhhhhhhhhhhhhhhhhhhhhhh
iiiiiiiiiiiiiiiiiiiiiiiiiii
jjjjjjjjjjjj
llllllllllllllllllllllllll
mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
nnnnnnnnnnnnnnnnnnnnnn
oooooooooooo
ppppppppppppppppppppppppppppppppppppppppppp
qqqqqqqqqqqqqqqqqqqqqqqqqqqqqq
rrrrrrrrrrrrrrrrrrrr
sssssssssssssssssssssssssssssss
tttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttt
uuuuuuuuuuuuuuuuuuuu

Finally, ctrl + v in a window and confirm to send. Here most of the time it doesn't ship. However, I don't know why, sometimes it sends. When it is not sent, the ON INPUT event is not triggered.