Photos backed up on iCloud get deleted when I delete them from my iPhone

Actually it’s not a real backup it’s syncing the photos between the iCloud and your devices, you can download them/copy them and keep them as a backup on your Mac/PC/SSD external memory

Transfer photos and videos from your iPhone or iPad to your Mac or PC – Apple Support (UK)

this is explaining how it works

Set up and use iCloud Photos – Apple Support (UK)

KJaxon wrote:

I was told unless I go in the cloud and delete it. It never gets deleted they’re safe there, and now I’m finding out it’s all a lie

Yes, whoever told you that was wrong.

I recommend that you back your photos up directly to your computer. If you have a Mac, I'd suggest using the built in Image Capture application.