So, I’ve just come across this issue as well. Created a board with 118mm x 53mm, uploaded it to JLC, and it’s reported as 118.05 x 53.05. Hmmm. I know in practice it doesn’t matter a jot, but I’m pretty sure it didn’t do that previously, so a bit of investigation required…
First step, I uploaded a different board from a previous project, and JLC accurately reported the dimensions. Recreating the gerbers from this previous project using the latest version of KiCAD and it still worked fine. That kinda ruled out a KiCAD issue.
I then started to look at differences in the individual gerber files. Long story short, it turns out that if there are NO drilled holes on the PCB, and the .drl gerber file has nothing in it other than the default text, JLC reports the dimensions with the extra 0.05mm line thickness. This doesn’t happen if .drl file has any drill holes defined.
So, the solution for me was to stick a couple of vias on the board, and that ‘fixed’ the problem.