I was using the single git repo, and setting the build name using:

 ${ENV,var="POM_VERSION"}

This was giving me the correct version string gy the versions:set target:
naven goals:

-DBUILD_NUMBER=${SHORT_BRANCH}-${BUILD_NUMBER} clean versions:set

ie:

3.48.0-develop-87

After switching over to the multiple-scms-plugin and adding a second git repo for a gradle project, the build name was now:

3.48.0-SNAPSHOT

even though there was still only a single maven project.
It had moved down into a subdirectory as I needed to do so in order to pull in another git repo, each in their own dir, but no changes to the maven build other than that.

There were no new maven targets added. AFAICT it's related to the addition of the second git repo or the switch from a single repo to the multipls-scms-pluin.

I added a shell exec immediately after the maven versions:set:

echo post versions:set POM_VERSION=3.48.0-SNAPSHOT

which logs:

post versions:set POM_VERSION=3.48.0-SNAPSHOT