my uname -a says
NetBSD 9.0 NetBSD 9.0 (GENERIC) #0: Fri Feb 14 00:06:28 UTC 2020 mkrepro@mkrepro.NetBSD.org:/usr/src/sys/arch/amd64/compile/GENERIC amd64
it was the one on the netbsd website main page, not the daily-build one. but still, had this "problem" on two different machines, one intel i5, and another low power celeron.. and it happened also in netbsd8. both my machines worked just fine, i experimented with it - i opened firefox and loaded crapload of tabs just to see what will happen when it fills up all of memory, but the system never slowed down.
i dont know if its a memory leak or a cache-thingie, but it irked me the wrong way so i put openbsd on my main machine and went with that..