After being frustrated with BR Slim Sport strap length adjustment creeping over time and some experimentations with strap/buckle routing I’ve found that simply rerouting the strap in the “keeper” clip makes length adjustment staying in place.
I hope the below pictures will help visualize what I mean.
In stock configuration the adjustment strap is tucked under the clip on the keeper buckle. Pic 1
But if you move it into the same slot as base strap and closer to adjustment buckle it will create enough friction to keep length adjustment from slipping when moving camera around. Pic 2
You lose “quick” adjustment capability which is not that useless considering that adjustment buckle is on the back but get more secure in place locking of length adjustment.
Hope that'd help others who had similar problem with BR straps.