This glitch has happened ever since 1.8.2 as far as I remember, and it causes non updated lava that generates in layers along with gravel. This world generation glitch usually happens in very small areas, and it can happen at various levels near gravel in the Nether (and not just near the lava ocean like in the following example). This doesn't happen in soul sand valleys or basalt deltas. To reproduce, use the seed 7912444071293930782 and run the following command:
/execute in minecraft:the_nether run tp @p -64 32 -62
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This bug has been apparent ever since I discovered the Nether, and I've been trying to reproduce this for years, so I'm very glad I found this extremely rare but disturbing bug in time for the Nether Update. If anybody can find the seed and coordinates of more examples of this bug, it would be very appreciated so that I could find more information on this odd world generation.

This happened to me
Edit: the seed is 3758706049760123925 the cords are -10 y -24
I'll add a screenshot
This currently happens in the Nether Wastes and Crimson Forest biomes. Presumably happens in the Warped Forest, but probably doesn't happen in the Soul Sand Valley (due to gravel beaches).
Edit: I have never seen this happen in a soul sand valley, and deconfirmed it for basalt delta as biome distribution was changed and a basalt delta made the old seed obsolete. I updated the ticket and changed the seed and coordinates.

Common bug in 20w18a
This does actually happen in Basalt Deltas, but only at the edge of the biome.
As said on the Minecraft website:
Fixed issues with the Nether surface generation, causing issues like floating lava and crimson forests without nylium
I have tested this, it happened in 1.16 Pre-release 8 but is now fixed in 1.16 Release Candidate 1