@Cyrinus
Hmm. That's odd considering that even the unfertilized evolution fruit plant has a watering interval of 14 hours and 30 minutes, and from what I understand, plants start losing grade each hour past the interval, so you would've had to been gone for 15 hours and 30 minutes to lose a grade. But your plant has a water fertilizer, so this sounds like it may be a bug. I'm not sure if GMs can check on this sort of thing, so I'll ping @Takkju.

The only advice I can think to give is to test it one more time. Water it, and then if you can, set an alarm for 14 hours and 30 minutes from that moment. If it loses grade again, then go ahead and create a bug report.