I seriously doubt many hitters have the prolonged and horrid slumps JBJ has, and in the last 3 years- all have been to start the season.
.800 hitters may slump for 3-5 weeks, but their slumps might be at .550-.700 not .409 to .537.
BTW, JBJ was an .800+ hitter from 2015-2016 (.834 over all 230 games), but even then, he had some horrid slumps:
2015:
.384 in first 21 games! (.962 last 53 games)
.608 in first 27 games
2016:
.586 first 16 games (.861 last 140 games)