Wintersday slot in Black Lion chests
Not a bug. The seasonal item stays until the next ‘season’, whether that be Living World release, Festival, etc.
If you don’t care for the seasonal item(s), you will always feel the chests are worth 2/3.
Seasonal items are a fairly new thing and do indeed take up 1/3 space. But they are not going to go away. It was cooler, when they were put in a permanent 4th slot of a time (around Halloween), but then this was changed. Boosters have been permanently removed, but to me it seems that useless stuff seems more common a drop these days than before (for instance, I haven’t seen a single home instance node in ages, so their prices have increased).
See this earlier thread on the same topic:
https://forum-en.gw2archive.eu/forum/game/gw2/Wintersday-gift-inside-blacklionchest/6462317
short story: ‘seasonal drop’ in BL chest is linked only to the start of festivals, not the end (at least, so far)