Grass doesn't spread from vanilla grass blocks to regolith

[saltyseadoggo]

Greetings! I constructed a bit of terrain using siltstone regolith outside of a little hobbit hole in my survival world, only to find that the outside grass wasn't spreading onto it at all! My partner gave it a test in a testing world with a high random tick speed to confirm that grass doesn't spread at all from vanilla grass blocks to any type of regolith.

The following picture of non-grassy regolith on a high random tick speed. Recreating this setup in a The Void superflat preset world and running /gamerule randomTickSpeed 99 will let you replicate this experiment. In the same setup, grass spreads quite quickly onto vanilla dirt.

[saltyseadoggo]

So I just read the bit on the CurseForge page about how "Regolith grass can spread to nearby regoliths/dirt only if there is lichen on it." However, adding lichen to the above experiment on top of the grass blocks and/or the regolith blocks still doesn't allow the grass to spread, even if the lichen is hydrated.

The below image is the above experiment, on random tick speed 90: