Synchronet Classic Shell causes menu loop with response "unrecognised command" on a legacy Wyse 520-ES dumb terminal

When loading main menu (and some sub menus) - Wyse 520-ES terminal causes a menu re-load with the response from the BBS of "unrecognised command".

Suggestion from DigitalMan:

for example, try removing the @-code stuff to start with @INCLUDE's and @HOT's or, try one at a time. Just keep removing stuff until the problem goes away. And then we'll have some clue what the cause is. If it's the @HOT codes, then it is mouse related.

Issue seems to be with head.msg, in particular this sequence shown in unix vi:

^An^Al^A<80>^An

Removing the ^Al fixes the issue.