A typo in the build script of AOSP-based custom ROMs for the Google Pixel and Pixel XL smartphones is reportedly causing the photo uploads from Google Camera to count against the Google Photos storage limit. The bug, however, is fixed now and users running custom ROMs on first gen Pixel phones should be able to use the unlimited Google Photos storage as usual.
https://www.notebookcheck.net/Typo-in-custom-ROM-code-breaks-unlimited-photo-backups-on-the-Google-Pixels.315530.0.html