The tag is present in Lintian version 2.104.234. That is the most recent version we know about.
The debug package appear to be containing debug symbols for a "Multi-Arch: same" package, but the debug package itself is not "Multi-Arch: same". If so, it is not possible to have the debug symbols for all architecture variants of the binaries available at the same time.
Making a debug package co-installable with itself is very trivial,
when installing the debug symbols beneath:
dh_strip does this in debhelper compat 9. Otherwise, the expected location of the debug symbols of a given ELF binary can be determined by using:
readelf -n <binary-elf> | \ perl -ne 'print if s,^\s*Build ID:\s*(\S\S)(\S+),/usr/lib/debug/.build-id/$1/$2.debug,'