My guess is that someone forgot to change the flight number on the transponder, so what you're looking at is the path of one plane over a couple flights.
ADS-B data isn't linked to the transponder code - each plane has a serial number assigned by the ICAO. So this is the simply one plane flying several different routes.
the flight number is what was left unchanged between several flights, thus they appear as one.
leaving transponder code unchanged after request by ATC is a serious violation.
This seems to be a not too uncommon occurrence based on what I've seen via the site. I find it a bit discomforting that this happens at all.
There are multi-leg flights that share the same flight number. I recall of a particular AA flight that has the same flight number from JFK-ORD and subsequently from ORD-MIA (or something like that).