Sailors maintenance is 2% of the sailor cost of the ship per month. Since 1.31.0 the sailor cost for ships has changed and now late game ships use more sailors than earlier ships. In the case of heavies, it is much more than in 1.30. I looked at your save, the threedeckers you have need 900 sailors to build and 18 sailors per month to maintain. If you move your fleet with 62 threedeckers(900 sailors), 59 archipelago frigates(180 sailors) and 74 trabakuls (90 sailors) out of port, it costs
(62*900+59*180+74*90)*0.02=1461 sailors per month and that is exactly the number which I see in your game if I put all other ships in port. That will deplete your reserves rather quickly. But nothing of this should have changed in 1.31.4. It is the same as in 1.31.3 or earlier 1.31.0 patches unless there was a bug in earlier patches which reduced the maintenance for some reason.