Using bone meal on the warped/crimson nylium causes sprouts and roots of the nylium's type to grow. But if you have warped and crimson nylium next to each other and use bone meal on one of them, their sprouts can spread to each other and other grass blocks despite not growing there naturally.
(See pictures for example)
Nylium plants can spread to:
Podzol
Grass Blocks
Soul Soil
Other types of Nylium
Can confirm in 20w49a.