I had this happen with my 6800 series crank (4 years old), I had noticed the spider arms seemed raised relative to chainring but it did not occur to me the crankset was failing. There was a clicking noise which led me to replace the bottom bracket, change the pedals to no effect.
Another indicator was my power numbers seemed to be going backwards and I was failing even more VO2 workouts than normal (I’ve always struggled with those 120% especially the longer efforts)
Wonder if there is a correlation to cranks used on trainers in erg mode and the spiral of death?