Grout and/or thinset flushed in septic system
I would call a plumber that has a snake with a camera to inspect the drains to see if the thinset or anything else has plugged or partially restricted the drains. A partial restriction can result in debris plugging that line from time to time and certain types of tissue getting caught by a rough surface. It would be better to fix the problem now than having issues at a later date.  my 2 cents