The grass won't spawn after proper material being applied on the landscape. Even after being forced to be spawned (by reloading the level, for example) the grass will disappear again every time the sculpt tool is used. Everything works as expected if Edit Layers is disabled.
This was reported and tested in 4.24.3 (CL-11590370). This was reproduced in 4.25 Preview 4 (CL-12366721) and Main 4.26 (CL-12620833).
Results: The grass won't spawn over the landscape after the material is applied and/or disappears when the landscape is sculpted
Expected: The grass should spawn over the landscape right after the material is applied or the sculpt tool is used.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-91593 in the post.