The tag is present in Lintian version 2.104.234. That is the most recent version we know about.
This file starts with the #! sequence that marks interpreted scripts, but it is a bash completion script that is merely intended to be sourced.
Please remove the line with hashbang, including any designated interpreter.
Refer to https://salsa.debian.org/lintian/lintian/-/merge_requests/292#note_139494 for details.