Please revert the fonts change
The change of the monospace and documents fonts has caused applications to break on flathub.
We now ship a runtime that says the monospace font is "Source Code Pro 10", which is a font that is neither contained in the runtime, nor present on most users systems. And the fontconfig configuration on their systems does not know how to fallback to a different monospace font for this.
My suggestion: Revert this change, and go back to using 'monospace' as the default. It is a well-established alias, and all systems will provide some working font setup for it.