I’ve been having this problem for a few versions of Ubuntu now. Had 18.04 and upgraded to 20.04 on my Asus X552cl laptop.
The laptop is running 24/7 and only needs a restart because of this issue where when it starts up the system registers the full memory installed (12GB showing as 11.5GB in Ubuntu) but sometimes things will lag and then it only shows around 4GB installed. OS is of course 64-bit and it’s not like it happens at specific times or specific reason that I can easily recreate it.
Like I said I’ve been using it a while now and it’s more annoying than a real problem, but I’d like to try and find out what is the cause.
Any more details required please let me know and I’ll post them.
My first thought was VLC causing the issue because the app will play during the night and next morning if I close it the background process stays active and some times the issue would occur then. Moved over to totem which does not have the background process issue but memory will still sometimes vanish.
Then I thought it might be sleep mode as our power gets shut down due to loadshedding, however it’s also not consistent as only a few times I’ve noticed missing memory when returning from sleep mode.