version:set-property doesn't seem to pick up -Dproperty and -DnewVersion user properties

Platform: debian 13 "trixie", amd64, maven 3.9.10, java 21.0.4, versions-maven-plugin 2.18.0

I am trying to change the bang-servlet.version property in this pom
 https://github.com/steinarb/bang-bompom/blob/master/bang-pom/bang-app-pom/pom.xml#L31
using this script:
 #!/bin/sh
 SERVLET_VERSION=$(cd $HOME/workspaces/servlet; mvn -q -Dexec.executable="echo" -Dexec.args='${project.version}' --non-recursive exec:exec)
 echo $SERVLET_VERSION
 (cd $HOME/git/bang-bompom/bang-pom/bang-app-pom; mvn versions:set-property -Dproperty='bang-servlet.version' -DnewVersion="$SERVLET_VERSION" -DallowSnapshots=true)

But the property in bang-pom/bang-app-pom/pom.xml isn't modified. The
file is unchanged.

I added the -X flag to the mvn command and discovered that <property> is
bang-bom.version and newVersion is 2.1.15-SNAPSHOT (instead of the
3.0.2-SNAPSHOT of SERVLET_VERSION):
 https://gist.github.com/steinarb/f186b038f04ebaf4685af5f04b1dd0ab#file-set-version-log-01-L194

I wondered if I was using the wrong form for the user arguments, but I'm
not, according to the documentation:
 https://www.mojohaus.org/versions/versions-maven-plugin/set-property-mojo.html#property
 https://www.mojohaus.org/versions/versions-maven-plugin/set-property-mojo.html#newVersion

So what am I doing wrong?

Various things I have tried, that haven't worked:
 1. Adding a dependency using the property as version in dependency
    management of the pom I'm trying to modify
 2. Referencing the above dependency managment dependency in the
    <dependencies> section
 3. Removing the dependency management and just having a dependency in
    <dependencies> using the property in <version>
 4. Running the "mvn versions:set-property" command from the top pom
    instead of the pom with the property I'm trying to change
 5. Add "--non-recursive" to the "mvn versions:set-property" command

But, as said, none of those had any effect.

The bang-pom/bang-app-pom/pom.xml file stayed unchanged.

All ideas and hints and suggestions are welcome!

Thanks!