This is related to the fix of MC-215939, which was later reverted by MC-226926, probably.

Emerald ore generates at very low frequencies on high altitudes compared to the frequency at how iron and coal ores generate. This was fixed at some point on the 1.17 snapshots, but then was reverted later when world changes got postponed.

Theoretically, emeralds should spawn even more often than coal and iron at relatively high heights as per the ore distribution graph, but doesn't seem to be the case since the development of 1.18 snapshots.

Examples of this in a experimental snapshot 4:

In 21w37a, it's about the same:

Might be worth noting that in bedrock edition, seems like emerald generates on blobs of 0-10 ores, which would make it a parity issue, but i'm not so sure about this.

How to reproduce

1. Create a world and try to find a big mountain biome.
2. Remove all stone and non-ore blocks using the /fill command.
3. You can see how little emerald generates compared to iron and coal.