I haven't rebooted. I added a swap file with higher priority instead of making the original one larger precisely to avoid rebooting. I don't dare interrupt Bitcoin Core.
Discussion
Understood. It seems entirely likely to me that without rebooting, the alterations you are making to the swap environment aren't even actually applied yet (until reboot that is). The growth in RAM usage could simply be a factor of Core's memory requirements growing as it progresses through processing its history of transactions.
At this point, it's all voodoo, mate 😂