I lost count on how many times this exact depo cause me issue because it backfill everything with slag.

I change the red bar to remove if above x%, and some time pass and it is back to block all again.

After while of getting angry with it, I finally reallize, every belt event that "relates" to the depo make it reset, that make no sense.
I do have the setting "Disable logistic for newly placed building" enabled, but that is not a newly placed building.

I finally realize what was happenning when I update the input belt that was bringing slag, the belt was not keeping up with all furnaces, the slag was floing into the depo, trucks were moving the excess away, but the belt was too slow and machine were cloged. As I update it, I just watch it get full and blocked, and when I clicked on it, the settings to export was yet again reseted back to not export.
It should not reset the depo config when you connect a belt or simple update a belt to it, or give a way to configure it to not change, specially when updating it.
With that being said, I still not able to make it happen at will, it some times happens, if I manage to record it happening I'll update here