I use the latest angular and material (15) but I want to use also flex layout.
I have the same problem. I tried to port an Angular 14 app to 15 today and I faced several issues. A typical Angular/Flex Layout/Angular Material app, that also uses theming to be more precise.
I decided to go stay on version 14 until it's possible to upgrade without issues. However, if you want to be able to upgrade to later versions in the future you should move away from Flex Layout.