Have you tried updating older trees with the JVC /Harold Minky method ?, It improves billboard trees to a large extent, but in trs2019 light from certain angles does give some unexpected affects. As usual, every iteration of trainz seems to render some previous tree assets useless. There are no plans to fix this problem, unless NV3 have changed their minds , its only a matter of time before all of them are going to either look incredibly outdated or they just wont work at all.
BTW for 3d creators, Cayden has released some good tree tutorials to use in blender which are online.