I run Roon on ROCK on an intel NUC.
I have not seen signs of endless memory growth - physical and managed memory (or threads and handles) as far as I can determine from server log seems to have been fairly consistent over the past 6 weeks or so from a very quick skim through. Some operation (lots of browsing) will push it up a lot for a while as may be expected.
Memory use can change alot depending upon what you have been doing (easily suddenly consuming another 1GB or so). I cannot comment on how long it may take for memory to get released again after such operations as that is something only Roon developers will know and I have not been been hunting through logs to that degree - only enough to see that day to day/week to week I am not seeing consistent growth.
1.8GB doesnt seem out of place.