Glow berries can generate on the floor of the cave in certain situations, creating buggy glow bushes. I believe this only occurs on narrow ledges but I could be wrong as occurrences are very rare. Edit: It looks like this bug can occur on 1 block thick surfaces with a space below them where glow vines could normally generate.

Examples:
Seed: -2048782000730431984
Locations: 171 -21 24 in the open on a ledge
1278 7 1953 in the middle of a ravine. Not on a ledge but there is an open pocket 1 block below it.
693 18 3210 at the entrance to a smaller cavern

However
If I may be so bold as to state my opinion, the floor of this biome is too dark, and leaving our fabulous axolotl friends in such horrid darkness would be a travesty indeed. The areas where this bug occurs look so much nicer once you light up the berries, and I was excited when I noticed this at first because I thought you had added glow bushes. This bug inadvertently made some places more lively, and I think this bug should stay in a more clean form.