I want to use a specific palette to make it strictly index color.

[Ofnuts]

Two explanations:

1. There is a bug, in which case you can report it (but with the actual image and palette)
   
2. Your expectation of "closest color" is not the same as Gimp's. This wouldn't be surprising since looking at the comments in the source code (see here around line 31) Gimp  doesn't match colors in RGB but in CIE L*a*b space (and the comment at line 22 hints that in Gimp3 this will be done using the linear and not gamma-encoded values).