There seems to appear no slag! The char is heavily loaded with soot and ash. Maybe the heavy soot content is hindering ash to sinter into slag bits?
The “grate” is actually a perforated sieve plate, only letting down small particles and grains.
When emtying the bunker and heavily banging the sieve from below, a vast amount of soot and ash is “flowing” down. The main part is soot and small char grains.
The sieve has “full diameter”, meaning that very little can drop over the edge. So this is not a blow-through grate.
Full diameter to the outer mantel “skin” cylinder where the ash hatch is.
It then takes ~1,5 hours of open road driving to restore normal reduction conditions. Until then, the gas appears weak, not giving normal top speed.
Egg-sized fuel gives the best “Autobahn” speeds >110km/h, but cigarett box-size works well in town and mixed driving.
Conclusion: A proper grate will (at least as a narrow-diameter blow-through type) blow down the soot and small char, leaving the ash flakes free to melt into growing slag bits…
Kristijan may avoid slag melting by increasing the restriction diameter in small increments…