This may be expected although I’m not quite sure why and would like to have further feedback to have a better understanding and aid other community members if they happen to have the same doubt. I am noob to Ethereum and Infura, so question might be silly.
STEPS TO RECREATE
Use FF 68.0.1 (64-bit)
New Project > View Project > Keys Endpoint > Rinkeby > Save Changes
Go to dashboard and view project once again, Keys Endpoint will show Mainnet instead of Rinkeby which was previously selected and saved.
Is this expected? , Why?
Blockgeeks videos: Basically learning here as a start point and the video does show Rinkeby as a fixed endpoint at minute 6:22. Videos might be a bit outdated but I would not have expected much to change:
Infura docs: Checked the following link InfuraDoc