Seems to be a more recent development or has gotten worse recently. I have seen it a couple of times... not sure if this is just a timeout on inactive pages, battery use optimization, or if another process is hogging resources... going back to a page then back to tabs seems to bring them back from the dead.
It also seems that this may happen if you get an ill-behaved page open. was just having trouble closing a page I had opened but not viewed and decided didn't need. I frequently do a search and open several pages then walk down the line unless I'm satisfied with what I found and close the rest.