I tested it with a dwarve race, whose strength modifier is currently +2, it also has durable trait already, but it gained only +8 in health after levelling up to level 2. But the levelling to level 3 and after, it gained only +6 health (I did not increase the strength modifier by adding strength stats)
I think this is a bug on the strength modifier where it only increases the health on the first level up. The second levelling-up and after, somehow the strength modifier isnt counted anymore.
Cheers, bigbear.