The new mIRC 6.2 has something which might rather be a fix to an old bug (if it was a bug at all), or a bug by itself.

In old versions of mIRC, if you would have created a window with tabs (window -t10,20,30,40), and then add a line:

//aline @window 2First $chr(9)  Second $chr(9) Third $chr(9) Forth

Then the 2 would only affect the tab that it was set on.
In fact, all wrapping would affect only the columns they were set on.

With mIRC 6.2 it 2 affects all that comes after it (it is not being stoped by the $chr(9) )

This would cause incompatibilities with older scripts which counted on $chr(9) breaking the wrapping.
For example You coded something that you have expected that color would return to Black after the $chr(9)

Another example is if you wanted a line to be bold then you would do:
/aline @window $chr(2) First $chr(9) $chr(2) Second $chr(9) $chr(2) Third $chr(9) $chr(2) Forth

But now this is not the case, and this would cause the columns to have Bold No Bold Bold No Bold

All I saw in the whatsnew.txt is:

123.Added aline/iline/sline/etc. -t switch, forces a re-wrap of all
lines in a window that have not yet been wrapped to the current
window size.

This might be related (It talks about wrapping the whole lines), or maybe it is the cause of the bug.....