The /locate biome usually finds biomes, even cave biomes, except for the deep dark biome. There are some chances when locating the deep dark biome would result in the wrong coordinates.
For example, we're going to be using 3 random seeds to test (w/ Java as a comparison):

Seed #1: 565463436
BE coordinates: -186 104 0
JE coordinates: -676 -34 920

Seed #2: 65161496154
BE coordinates: -1184 128 -992
JE coordinates: -90 -17 -2249

Seed #3: 3216549867
BE coordinates: 32 121 -352
JE coordinates: 536 -36 -522

Seed #1 map and corresponding coordinate:

Seed #2 map and corresponding coordinate:

Seed #3 map and corresponding coordinate (w/ in-game screenshots):

From all the seeds that I got, seed #1 and #2 on BE give the coordinates to the outer edge of the deep dark biome boundary, instead of within inside of them. Seed #3 on BE gives completely wrong coordinates where there is no deep dark biome at all at the location.

How to Reproduce:
1. Create a new world (preferably with spectator mode for a quick check underground).
2. Locate a deep dark biome using /locate biome deep_dark command.
3. Switch to Spectator Mode.
4. Go to that coordinate that the command output gave.
5. Go underground to see if the deep dark exists.

Observed Result:
Sometimes the coordinates send you to the correct coordinate, but in some cases, it sends you to the wrong coordinates where there is no deep dark biome at all, despite what the command output says.

Expected Result:
The /locate biome deep_dark command should always give the correct coordinates, like what Java does.