Because you're using an $identifier where mIRC expects a /command, so mIRC tries to execute the return value of the identifier as a command.
So if $tip() returns 1, then mIRC will try to execute /1
To fix this, use the /noop command before $tip():
on *:TEXT:*:*: {
if ($me isin $1-) {
noop $tip(Name,Highlight, $+ $me Je bent genoemd in $chan om4 $timestamp ,7200)
}
}
/noop simply does nothing. So it lets you evaluate the contents of whatever you pass to it without doing anything with the results.