-vmargs Goes Missing During Tycho Build - eclipse-rcp

Question

I've a problem with my Tycho build. I've got a standard RCP product that starts the application. And a Tycho configuration like this:
<plugin>
<groupId>org.eclipse.tycho</groupId>
<artifactId>tycho-p2-director-plugin</artifactId>
<version>${tycho-version}</version>
<executions>
<execution>
<id>materialize-products</id>
<goals>
<goal>materialize-products</goal>
</goals>
</execution>
<execution>
<id>archive-products</id>
<goals>
<goal>archive-products</goal>
</goals>
</execution>
</executions>
<configuration>
<products>
<product>
<id>${project.artifactId}</id>
<attachId>${project.artifactId}</attachId>
<archiveFileName>${archiveName}</archiveFileName>
</product>
</products>
</configuration>
</plugin>
This is the entire configuration, and it's probably the most minimalistic one possible.
Which makes what happens during the Tycho build even weirder. The product file defines the following program arguments:
-clean
-persistState false
-vmargs --add-modules=ALL-SYSTEM
After the build the -vmargs is missing, so the INI file looks like this:
-startup
plugins/org.eclipse.equinox.launcher_1.4.0.v20161219-1356.jar
--launcher.library
plugins/org.eclipse.equinox.launcher.win32.win32.x86_64_1.1.551.v20171108-1834
-clean
-persistState
false
--add-modules=ALL-SYSTEM
This means the application does not work with this file, because --add-modules somehow needs -vmargs to work.
I have no idea where to even start debugging this problem. Why does Tycho remove the -vmargs argument and how do I prevent this from happening?