I have an iPhone 12 Pro Max and even on Wi-Fi, I find Photos not syncing photos to iCloud because it says it’s saving power. I can turn this off, but I have to do it many times over.
I am not in Low Power Mode, nor have I found any settings in Photos nor Battery to control this behavior.
I saw an old post with the same issue below, but there have been no solutions.
Everything will clear if I Resume updating. Surprisingly, I think my whole library is intact without any corruption. It’s just that after taking a new picture, it’s not sending it to iCloud until I resume it.
IME, Photos tends to be pretty power-conserving regardless of battery percentage. Best way to get it to upload (aside from hitting “Resume Updates” is to leave the phone connected to power.