What is "Lost RAM" in dumpsys meminfo?
Lost RAM is TotalRAM - FreeRAM - UsedRAM. That means, it is the difference between the RAM usage that Android is able to compute and the actual available RAM. 

Sometimes it can even be negative, due to issues where RAM being shared across processes is counted more than once. Drivers are mostly blamed for that. There is a memtrack HAL for them to report their memory accounting to Android, but some of drivers just doesn't do it correctly.

If you see a large "Lost RAM" value (which isn't the case in the original question, but is what Wang is talking about), this is probably due to a bug, it can't be reclaimed, unfortunately. You can look into /d/ion virtual directory on your device (probably a rooted device is needed), and examine reports in it, looking for "orphaned" allocations.