build.number suddenly not being set

We have a number of projects in which we set the build number from within MSBuild. This has been worked great for the past 2 years. Suddenly this has stopped working across all projects and I'm at a loss as to why. We are running TeamCity 2021.2.3 (build 99711).

This is how we set build.number from MSBuild:

<Message Text="Parsed version is $(OutVersion)" />
<Message Text="##teamcity[buildNumber '$(OutVersion)']" />

In the build logs I see:

10:28:14             Parsed version is 1.9.8129

Which means the next line is being output.

However build.number remains the text "build.number", as can be seen in the title of the agent build log and any artifacts produced:

Application_x64_build.number.zip

What I've tried:

• Dumping agent threads
• Restarting TeamCity server
• Restarting machine
• Clearing all caches

Has anyone else had this problem? What else should I be looking at? Is there any way to downgrade to a previous version?