I have no idea why but for me installing FPCamera.v7.00.04.0007.4.0.apk uploaded by @hirnsushi worked just fine on CalyxOS. The camera app seems to work, I can open it, take pictures and videos, change settings, switch to the gallery and so on.
I am running CalyxOS 5.6.0. Before attempting the install, l had already installed
- the previous FP camera version (which I rarely used because I preferred gcam and it always crashed when opening into photo mode, only Pro mode worked via the launcher shortcut)
- CalyxOS GcamPhotosPreview immitating Google Photos