The text file
gzdoom/wadsrc/static/zscript/statscreen/statscreen.txt
is set to use strings called “$ENTERING” and “$FINISHED”, located in the language files, in intermission screens between levels in Heretic. However, these strings are named incorrectly in the language files, instead being written as “$WI_ENTERING” and “$WI_FINISHED” for some reason I’m unaware of. After renaming the original script, the ingame text shows up through what is written in the language files, as intended.
On a miscellaneous note: in GZDoom, the text between levels in Heretic says “Entering:”. In the DOS version, it says “Now entering:”. This is accurately reflected in the English language file, though, and thus faithful to the original when displayed ingame.