Hello,
I hope my answer will help ! I don’t have this problem, but it seems that post number 15 contains a solution.
Anyway, I suggest you export your contacts to a VCF file (visit card file), preferably on the SD card if you have one, if not, do it on your computer. To do so, open the “Contacts” application. In the options (the 3 vertical points on the right top corner), select “Import/Export”, then “Export to a VCF file”. Save the file on your SD card or on your computer. If you encounter any contact loss, you can still download them from your VCF file, which won’t be deleted after the download.
But the loss of contacts is very annoying. To avoid this, you can also switch to Lineage OS, which my FP2 runs and I haven’t had any loss of contacts since. There is a full port (but still not official), with updates. You’ll be on Android 7.1.2. If you wish to do so, go there : LineageOS 14.1 with OTA updates for FP2. You can also look here : #dic:flash. But don’t forget to backup your data with TWRP on the SD card 