Hi CauserCancing,
Thanks for posting in Apple Support Communities. It sounds like you are using iCloud Photos to optimize your iPhone storage. As mentioned in Set up and use iCloud Photos - Apple Support, these photos are stored in iCloud in their original format so that you can access them any time from any of your devices. This can be dependent upon your internet speed and connection. The following is from that article.
"Your photos and videos are stored in iCloud in their original formats at full resolution. This includes HEIF, JPEG, RAW, PNG, GIF, TIFF, HEVC, and MP4 as well as special formats you capture with your iPhone or iPad, like slo-mo, time-lapse, 4K videos, and Live Photos. The photos and videos you capture with these devices are in HEIF and HEVC format. Depending on your internet speed, the time it takes for you to see your photos and videos on all of your devices and iCloud.com might vary."
Considering that this might be related to connection, try connecting your iPhone to another network if possible then see if the same occurs. If not, it may be best to have Apple Support check into this further. You can reach out from Get Support.
Kind regards.