I am DIY homeowner with a good amount of DIY plumbing experience. Installed a Fleck Cabinet unit 2 weeks ago. All joints sealed tight except one started seeping after a week. Retightened and it started seeping again this morning. 4 wraps nickel tape plus pipe dope. Tighten, wait 20 mins tighten again each time. See pic. It is stainless nipple into stainless street elbow. Our house was built in 70’s. Everything is old. No bladder on water heater. No check valve at entry. 80-85psi at entry. I believe as water is used and system pressures up/down, the flex connection is unscrewing the nipple. I marked it with a black line this time to see if I am right. Am I crazy? Is this a thing? If so, suggestions to remedy?