Then you didn’t do it properly or you didn’t set up the materials properly. you need to make sure to adjust the w/b levels to “amplify” contrast and such.
Read up on draw calls. I’m almost positive it’s having to make a draw call for each of the lightmaps; despite the trees all being duplicate instances of each other. The engine isn’t moving whole “boxes of paper” at once, it’s having to move “pieces of paper” one by one. So the character model analogy doesn’t work.
The main take-away here is that you’re persisting on using an inefficient method, complaining that the engine needs to be fixed or something and will not take suggestions; out of confirmation/belief biases. Whatever floats your boat though…