Hi Freddy, yes some stokers create clinkers,, most are not a problem at all.. On WNY's keystoker, the end of the stoker's bed is near the front of the stove, so when the rice-size coal would slightly fuse into a soft pillow-like clinker, it would not drop off the end of the bed into the ashpan.. and coal and ash would back up behind the clinker.. this is the only instance that I have read about where a clinker caused a problem.. Like you suggest , most just fall off into the ash pan.
Clinkers form usually only when the fire is burning very hot.. most anthracite coal's ash-fusion temperature is over 2500*, so you have to be burning pretty hot to start to form clinkers.. But some of this depends on the coal as well,, if the coal is very low ash, clinkers form easier. Higher ash content tends to insulate the pieces from each other so they don't fuse together.. at least this is my 'theory' why low ash coal fuses into clinker easier.
The AHS and AxemanAnderson boilers make clinkers too, but the are ejected with the ash into the ashpan, so they are not a problem.. The EFM underfeed type stoker will have some clinkers, but they just fall into the ashpan,, if they hang up in the firebox, they are easy to see and push off the edge of the firepot and into the ashpan,, so again, not a big problem.
Hope this helps answer your question.