On a recent caving expedition I found multiple orchids underneath the surface. I believe this to be a bug.
MC-148259
Blue orchid found in cave
Resolved
Duplicate
0
Jean-Luc Robitaille
(Unassigned)
Unconfirmed
Minecraft 1.14 Pre-Release 2
Retrieved
I doubt this is a bug, but is intended behaviour. Orchids generate in the swamp biome which you are in and can generate on dirt even underground. This happens in any biome that can generate grass, flowers etc..