I have the same issue on my iPad Air 4 and iPhone 12 Pro Max. I wiped them both to start "fresh" on iOS 15 and restored without a backup.
Before iOS 15 I had 102 GB free on my 128 GB iPhone with 4 GB "locally" stored photos (rest being in the iCloud). After restoring the iPhone, I have had only 64 GB free with Photos suddenly taking a huge junk.
Therefore I wanted to test something on my iPad and thus deactivated iCloud Photo Library and deleted every single photo from the iPad. HOWEVER, the storage still claims that I have 8 GB of photos on the device even though there was not a single photo on it. Not really knowing what to do about that, I activated iCloud Photo Library again and "Photos" is taking less space, however ... now I have a huge amount of "Others" instead.
No idea what is going on there.
Prior to this (already on iOS 15) I have noticed that my MacBook asked me to import thousands of photos from my device when plugged in, even though those images have already been on iCloud for years. Why was it even asking me to import photos when iCloud Photos was activated. I dont think it should even be asking since everything is supposed to be in sync automatically? (iCloud Photos is obviously activated on my Mac as well)