re: Weird plumbing issue in brand new house with tankless water heater

Might try opening all hot water faucets. If any restrictions, that would move it. Make sure all faucets are cleaned, the strainer part. Also for the shower head, remove the orifice.
I just had one installed for a friend, the tankless. The plumber I use knows more about those things. I you can’t get it to working properly, let me know.