This new Memory Management system came first in macOS Mavericks and resulted in my no longer monitoring the memory usage statistics altogether. It’s designed to speed up your computing experience by keeping RAM as full as possible with system and application cache files so they don’t have to be continually retrieved from your drives.
Apple guidance at the time was and still is to instead look at the graph in Activity Monitor’s Memory tab in the lower left corner called MEMORY PRESSURE. As long as it stays green, it’s working perfectly. Yellow may indicate the need to watch more closely. Red indicates a need to add RAM, if possible.