iCloud missing photos 9 month gaps

One month ago, I got a new iPhone with less storage than my previous iPhone and decided it was time to do the whole backup to iCloud thing (yes, I was late to the game). I had close to 13000 photos and videos on my old phone. I followed the instructions to get my photos & videos onto iCloud and paid for the required storage upgrade. This process took a long time.

When it was complete, I checked to ensure the upload was successful. I looked for the oldest photos, some in the middle, and the most recent. They were all there. Because there were 13000, I obviously couldn't check them all, but I did feel confident the upload was successful.

Next, I factory reset the old phone so it could be sold.

Today, 1 month later, I am trying to find a photo from 1 month before the transfer and it's not in my iCloud Photos. Panicking, I'm looking through iCloud Photos and realize there are large gaps of time missing. As a mom, I take photos daily, yet there are no photos between July 31, 2019-April 16, 2020 (9 months!!!!!) and March 2021-January 2022 (although I'm sure I checked for the most recent photos when I initially uploaded to iCloud).

Does anyone know where these missing photos are? Can I get them back?

Thank you in advance for any help or tips you can provide.