To reproduce 2 diffrent smeltable items are required

Insert a item then replace it with another as the furnace is running
it wont interupt the smelting (same as the iron furnace)
But unlike the iron furnace the electric one
will consume the 2nd item but return the output of the 1st one

Versions:
fabric-loader-0.4.8+build.159
fabric-api-0.3.1+build.208
TechReborn-1.14.4-3.0.9.165
RebornCore-1.14.4-4.0.11.140