It's because a lot of the scenery uses range animations based on the LOD settings in an attempt to improve performance.
Unfortunately this is a casualty of the LOD changes that I made in 2019.1 - which changes the ranges from absolute to relative - and uses different properties.
The scenery should never include range animations to manage detail because it slows down frame rate[1] these create unnecessary CPU load and mostly it's faster to just draw everything.
The fix is obviously to update all of the scenery XML files to remove the range animations - but this is something that is still a WIP.
I am currently testing a build that removes almost all range animations - but this is a bit of a drastic step and may make things worse.
The last possible solution is that I could make the current builds compatible with these incorrect scenery range animations - but I really don't want to do this as it's supporting something that is wrong.
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[1] I know that managing detail seems like it should improve frame rate - but it doesn't. In my tests I can get an extra 7 FPS over Paris by completely removing all range animations.