I’ve generally seen a pretty quick falloff on lead acid batteries when they go into rest. If you stop holding them at the 13.8 float and just let them rest, they drop to 13.5 or less within minutes.Typically stock alternators are set to about 13.8v , so let’s say you set the set point at 13.5V , fine
But when the engine stops you will tend to discharge the AGM until the cut off point is reached. AGMs hate partial discharges. It reduces their life considerably.
A better way is to establish a engine run stop signal and switch the Victron dc dc on off ( it’s has that ability )