Partitions - why do synthesis and fitter still run on Post-fit partition?

I have created a partitioned design containing 6 partitions.

My SOPC system is one partition. I want to avoid always resynthesizing it to save time when doing other things in my design, and I rarely change it, so I mark it as a Post-Fit Netlist in Quartus.

However, many times (not always), it goes back into this partition regardless and resynthesizes it, even though the changes I have been making are unrelated to it.

Why?

Thanks