Virtualmin GPL here, on a Debian 10.
I had the same issue on my previous Dedi, with an older Debian: the Dashboard and resources usage dashboard page gives vastly incorrect results, compared to what a "top" or "free -m" terminal command returns.
It's all in the attached screenshots (along with dedicated server hardware information).
Virtualmin dashboard: telling only 14% of the real memory is used, then telling 26 GB out of 31 GB are unused. Terminal, top or free -m: telling there's under 300 megabytes of free RAM.
Honest disclaimer, it's not my job at all, so maybe I'm making a gigantic confusion here, with cache or swap, I learnt it the hard way, the more obvious, the easier to miss.
But maybe it really is a hug, so here's for the issue creation. Have a good day!