symbol free, version Qt_5 not defined in file libQt5WebEngineCore.so.5 with link time reference

I'm trying to create a Flatpak for Subsurface, which uses the KDE Sdk. The build manifest for the Sdk is found here: https://github.com/KDE/flatpak-kde-runtime/blob/master/org.kde.Sdk.json and is basically a recipe for building Qt 5.9.0 and KDE 5.35.0 libraries.

The recipe and a patch for Subsurface:

https://github.com/Alexander-Wilms/flatpak-manifests/blob/master/org.subsurface_divelog.subsurface.json

https://github.com/Alexander-Wilms/flatpak-manifests/blob/master/subsurface.patch

With Flatpak installed, the package can be built with 

flatpak-builder --repo=subsurface-repo subsurface-build org.subsurface_divelog.subsurface,json

Then one needs to add the repository:

flatpak remote-add --no-gpg-verify subsurface-repo subsurface-repo

The application needs to be installed:

flatpak install subsurface-repo org.subsurface_divelog.subsurface

And finally, it can be run:

flatpak run org.subsurface_divelog.subsurface

For debugging purposes there's this command:

flatpak run --devel --comand=sh org.subsurface_divelog.subsurface

I built Subsurface just fine, but when I run it, the following error occurs: 

subsurface: relocation error: subsurface: symbol free, version Qt_5 not defined in file libQt5WebEngineCore.so.5 with link time reference

This seems very similar to this bug, which should be fixed in 5.9.0: https://bugreports.qt.io/browse/QTBUG-60565

But the maintainer of the Sdk suspected that there could be something wrong about the way QtWebengine is being built.