I don't think it's PRE_INPUT per se. I suspect the execution of PRE_INPUT is somehow triggering what happened my first observation, namely that the title reverts during the likes of ECHO FOO & DELAY 15.
Note that none of this (icon too) happens if the instance is started with start with the title being TCC.EXE's fully-qualified path (as in START "%@full[%COMSPEC]" /pgm %COMSPEC) which is seemingly Windows's default behavior.
I don't know why TCC is checking whether the original title is TCC's FQ path and behaving differently depending on the result. The behavior goes back to, at least, 4NT v8 (and I'd bet, to 4DOS).
The results are in keeping with previous observations. During ECHO FOO & DELAY 15 and whenever PRE_INPUT is an alias, the title reverts to the start-up title ... an empty string ... and TCC's icon doesn't show.
I agree, the icon business is a Windows thing. Whether the application-supplied icon is used seems to depend on the string in STARTUPINFO::lpTitle; you get the app-supplied icon if and only if that string is TCC.EXE's fully-qualified path. But it seems really odd that Windows would use the console's title to figure out what icon to use.