Hi, @janne, there are two possibilities that come to mind.
The first is that the Trusty build image is used for the older site. If so there is more information about how to switch to Xenial here.
The other possibility is that the repo itself hasn’t been relinked since the monorepo support was enabled in Xenial:
If so, click the Edit button and select the same repo to relink it under Site Name > Settings > Build & deploy > Continuous Deployment.
If there are other questions about this or if the solution above doesn’t work, please let us know.