The tag is present in Lintian version 2.104.328. That is the most recent version we know about.
The package is architecture-dependent, ships a cmake file in a public,
architecture-dependent cmake search path and is marked
foreign. CMake will be unable to find this file, unless it is installed
for a matching architecture, but the
foreign marking says that the
architecture should not matter.
Please remove the
Multi-Arch: foreign stanza.