All 10 single block varieties of flowers will increment 1 upon being mined, however all four Double Plant varieties are not incrementing upon being mined.
The following two criteria are verified as working for all 14 flowers:
minecraft.picked_up:minecraft.[ ]
minecraft.used:minecraft.[ ]
Environment
Scoreboard system (in Survival), in particular the following criteria:
minecraft.mined:minecraft.lilac
minecraft.mined:minecraft.peony
minecraft.mined:minecraft.rose_bush
minecraft.mined:minecraft.sunflower
Please check if that's still an issue for you in the latest 1.15.2 stable release.