Cory barbel erosion

This tank is approx 25 days old. One of my Cory panda lost all it's barbels, total erosion. This substrate is finely uniformed and very light for Corys barbels to be pushed around. Also, the substrate is clean and not ticker than 2cm. I can see the Malaysian Trumpet snails digging through the sand (literally since the sand is very light) at all times.
I am performing weekly 25% water change and cleaning the filter sponge. I have very good water flow + good surface agitation.
I can say for sure, the substrate didn't cause this erosion, and I am not sure what did.
I am feeding them with Krill flakes, Tetra Tabimin (for Corys) and Sera Vipachips (for Corys). Also with normal flakes (Vitakraft).
No ammonia nor nitrite. Remaining Corys seem to be doing well. One Cory panda dyed.
Total 4 Corydoras panda and 6 Corydoras melini.
Photo showing MTS digging through the substrate;

3 of the 6 small mystery fry are still living and growing. By the shape of their fins, I would say I have got some sort of Rainbow specie, not sure which yet (assumption). Time will tell.