Bottom bracket lockring keeps loosening up
If it's coming loose so soon after being actually tightened fully, then it's probably one of these 2 things:


The lockring threads are gone or in poor condition, probably suggesting they were that way before or as a result of the last time someone tightened it. The easy fix if this is all that's going on is a new lockring. Most bike shops should have a stray one. They're standard sized.
Some kind of binding or other bearing issue is going on inside the
left BB cup that's causing the spindle rotation to turn the entire cup in
the loosening direction. Normally how it's supposed to work is the
spindle is turning counterclockwise, which makes the ball bearings
orbit counterclockwise but individually spin clockwise, transferring
forces to the cup in its tightening direction. (Hence why ISO right
cups are reverse threaded, and also why Italian right cups are
notorious for coming unscrewed.) A retainer installed in the wrong
direction may be able to cause this, in addition to chewing up the
BB.