Why should anyone take a camera to take a screenshot or click on 'save image'?

They shouldn't, but that's what you said...

If you take a photo of it, it's saved into your camera roll

?

Beside that, I agree that its a security issue to have the plain text or QR code stored somwhere else than in the authenticator app itself. Inerently, trying to circumvent the introduced security by the uniqueness of the TOTP authenticator by sharing the key, one way or another, is a security issue by itself. That's why, I've recommended registering multiple independent authenticatior-keys, if possible, one per group member.