Flask app builds but deployment fails, possibly due to matplotlib or dependency management?

I am building and deploying this [1] [2] Flask app with App Platform. It incorporates two git submodules, each of them a python plotly/dash dashboard project. Build and deployment would work fine as long as I use native plotly plots. But a newer commit that additionally uses matplotlib included into the dashboard as per this [3] tutorial, fails, not during build, but during deployment. Locally on my own machine, it all works. What could be the cause?

If dependency management is a common issue: I am using normal pip + venv for the main app and one of the dashboards. The one with the matplotlib additions uses poetry when running standalone (both dashboards are designed to be able to run standalone). When being run by the main Flask app, it is this main app that provides the dependencies. (In other words, its requirements.txt contains more dependencies than strictly necessary for itself to run, but it also provides for its child apps.)

Any help is much appreciated.

Cheers

[1] https://github.com/andmbg/portfolio

[2] https://starfish-app-559vv.ondigitalocean.app/

[3] https://plotly.com/blog/dash-matplotlib