There is an issue where Static meshes in a particle system are not affected by decals, It is unsure if this is intentional behavior or not. This issue does not appear to be a regression
4.17.2 - CL: 3658906
4.18.2 - CL: 3794801
4.19.0 - CL: 3776583
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-53628 in the post.