The sync service may have its own algorithm that lets it choose when to stop syncing photos and videos.

The Low Battery Mode in iOS and iPadOS can eke out a lot more usable time when your charge runs low. It’s a big enhancement that turns itself on automatically. When Low Battery Mode engages, a number of background activities shut down, including sync and email checking.

When your battery tops 80 percent charge or you disable Low Battery Mode via Settings > Battery, operations should go back to normal.
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https://www.macworld.com/article/3511972/ios-what-to-do-if-icloud-photos-uploads-are-paused-due-to-a-low-battery-but-youre-actually-near-a-f.html