I think it's just a matter of perception because according to Fangraphs, the Tigers bullpen is 20th in WAR this year and they were 20th in WAR last year.
This year, the hitting and starting pitching are better than last year so the bullpen seems worse and it seemed like a strength but not really.
Individual performance probably skews perception as well. A few guys pitched very well last year but overall the results are very similar so far.