Some linedef actions are missing default settings in UDMF
From the wiki pages for linedef actions 14 and 15:
If the texture fields are left empty, MT_ROCKCRUMBLE1 is used as the default debris type.
If the texture fields are left empty, MT_PARTICLE is used as the default particle type.
These defaults seem to have been missed when porting the actions to UDMF, which leads to a lack of crumble particles in BCZ2: