When baking only indirect lighting, light sampling is skipped at the
first bounce. However, light evaluation is still done, so depending
on how the MIS weights end up more or less of the direct lighting
still ends up in the bake.
This is most noticeable with background lighting, but can also be
reproduced with e.g. point lights with a large radius.
Pull Request: https://projects.blender.org/blender/blender/pulls/108955
a783fe8198