I’ve noticed this behavior quite a few times now. I am running the latest Simlink 1.1.18.0916 and I only use it with MSFS 2020. In the current instance, my Windows uptime is 4 days and I have not opened MSFS at all during that time. You can see below that the working set for NavigraphSimlink.exe is reported as 13GB in Process Explorer.
I don’t know much more about the behavior at this point, though I feel like it’s always been a similar number whenever I’ve noticed it.