Splitting a blender object into parts changes reflections drastically

[kimden]

Quite often, it's useful to split an object in Blender into two parts — for example, when a part of the wall should be shown only in certain game modes, and in other modes it should be hidden, then one can separate the interesting part, make a new object out of it, and set scripting options to show it under certain conditions.

However, it breaks the reflections at least in case when the material of the object uses reflections.

Example - monkeys

Consider the following map (zip, 351kb). It consists of four standard monkey models placed as 2x2 grid, while top left and bottom right monkeys are split into the big top part and small mouth part:

The color of monkeys is solid 180 130 80, while the color of their gloss map is also solid 150 150 0.

If you try to add more light by using nitro and going into the monkeys, you can spot the difference:

SP debug > Toggle normals visualization clearly shows differences between the two bottom monkeys - you can see cyan circles at the sides above the mouth for the split monkey:

Examples - real maps

I have encountered strange reflections/shadows while making other maps before, as early as in 2022 (left picture), but also this year (right picture). In both cases, it's the same material used in differing parts, and the common thing for two maps is that a certain object had a part being separated for scripting reasons (for the left, the barrier that went to the left initially; for the right, the right half of the wall is another object).

Configuration

As always, I used my custom client, but I would be really surprised if it cannot be reproduced with the latest git 502fcf5, as I suppose the issue is there for at least three years.