I have discovered this article, explaining that due to a bug in maven, the build timestamp does not get propagated to the filtering. The workaround is to wrap the timestamp in another property:
<properties>
<timestamp>${maven.build.timestamp}</timestamp>
<maven.build.timestamp.format>yyyy-MM-dd HH:mm</maven.build.timestamp.format>
</properties>
Filtering then works as expected for
buildTimestamp=${timestamp}