Hi!
I have been playing the 1755 campaign with the latest patch and have sometimes experienced what you explain.
After much

i think i finally discovered the reason.
I'm pretty sure this happens on regions where your supply situation is "on the limit". Your units have supply, but are not 100% stocked because what you consume each turn is slightly higher than what the region can offer or becasue tehy get there with their stocks depleted and are slowly refilling them.
So the stack will have maybe 87% of their supply (check the tooltip over the supply icon on the top right of the unit panel), and next turn it will decreases slightly to lets say 84% or maybe increases slightly to 91%, but you don't produce enough supply to top off their stocks up to 100%.
From my experience, once a stack is completely supplied (at 100%), it will start recovering hits.
But as long as they still have some supply issues (even if they are 99% full they will not recover hits (but i think they can receive full missing elements).
Boston with a depot and a fort produce a lot of supplies, but if you have there a big land stack and maybe a big fleet (ships also need/consume supplies

) they can stretch their capabilities.
To approximately know how many units a place can feed multiply x5 the supply level number you get on the region tooltip and compare it with the supply consumption per turn of all the stacks (even navel ones) locate there.
Can this be your problem??
Regards!