Yes. I too have experience of straps "loosening", to the extent of ends becoming free, and therefore load not secured at all! Hence strap end mods.
However, this did not solve the "loosening off" of strap tension.
Imo, the straps cannot really be loosening, as i even tie off the free ends in a couple of half hitches.
I think the straps seem loose, because they cannot be really tightened hard against a soft load like a boat (without the risk of damage) the boat/trailer flexes on the road, the boat moves a little, therefore the straps then appear loose?
Regular stops, checks and strap tension adjustment seems all that is possible?
However, I have seen online some straps that appear to combine bungy with ratchet straps in an attempt to solve this problem. Have not tried them.