That’s worrying just because I have a suspicion that there are accordion pipes. So I just did a test. Plugged the drain & filled the sink to the top with water. Pulled the plug and ran to the basement. There is a strong gushing from the main pipe. So I’d say at least most of the water is going to the right place. So certainly it’s not a case where the sewage found a complete alternate path. The clog is in fact gone. Though there’s always a chance of leaks, which in this case would be into or below a concrete slab.