$?try is valid
$?=try is valid
The problem only occurs when there is an unbalanced number of "'s in the call.
mIRC will either take the first full word succeeding the "$?", ignoring an = if it is the first character. Consider the following:
//echo -a $?try
//echo -a $?=try
//echo -a $?==
Here's the important part: if it encounters a " it will ignore all the previous characters and start fresh until it finds the ending "
//echo -a $?try" harder" ; shows "harder" as the message
It looks until the end of the line (or next `|`) for the ending ". If it doesn't find it it silently errors out. It could show an error message for this instead, but it's certainly not a "bug". The behaviour is quite undefined and there are hundreds of places where syntax errors cause silent halting of scripts, in mIRC.
For completeness, this explains why your example works in the first case and not the second. ($?try it") is just "RESULTS OF $?try followed by `it"`, `it"` being an invalid operator.
The second one is that missing " case we were just talking about, which is why its problematic. If you had tryit without a space, both cases would fail.