Correct. Yes it is very unfortunate. However this issue is not exclusive to this mod. This bug also affects IO’s own levels (all examples have unmodded lighting):
Example from Wrath (ignore the HUD, was testing some mods):
Issue affecting multiple people including @Kevin_Rudd
As well as in Hokkaido (unmodded):
https://www.reddit.com/r/HiTMAN/comments/librim/hokkaido_is_a_lot_brighter_than_normal_in_the/
In the case specifically of the Classic Pack, it is because HITMAN 2016 did not have a boxreflectioncache entity, did not exist at that time or at least implemented in that way. So when removing it the lighting is much more accurate to 2016 but it breaks when loading after another level.
The Vanilla+ pack is unaffected however as it contains the boxreflectioncache entities.