Servant. The drop in quality and atmosphere between season 1 and the following 3 is beyond baffling. Goes from psychological thriller to sitcom very quick and I felt sorry for the cast by the end.

Yellowjackets. Extremely tight, well plotted season 1, with an amazing cast. By season 2 the writers had dropped the ball so hard, cast members started complaining publicly and/or just leaving the show. It's probably the most egregious example I can think of of writers promising a 5-season plan while so clearly and unapologetically making shit up as they go.