This is definitely a major bug - the only reason it isn’t getting more upvotes is glider pilots are so lost with the current implementation of rising/sinking air in MSFS it’s hard to know where to report what. The entire design of lift/sink is a remarkable disconnect from anything we’d recognise.
It seems likely that the flawed implentation of “ground lift” is conflicting with the implementation of “ridge lift” in that the “ground lift” implements sinking or rising air over every ground type, and “trees” is a ground type that probably generates sink. The scale of the sink/lift generated by this “ground lift” is greater than anything else implemented in the sim (probably “cloud lift” and “ridge lift”) effectively negating the value of those other lift types.
FWIW we (glider pilots) use a necessary workaround for the broken “ground lift” in MSFS by setting all flights in the coldest winter months, with a departure time typically around 10am. This has the effect of reducing the strength of the “ground lift” which also reduces the strength of the “ground sink”. This means the only lift available is ridge lift, so gliding flights are limited to that, but that is the best we can do as of SU11.