For whatever reason a valid blast furnace turns into an entity every 0,5 seconds or so

proof (you can see the blue lines):

Now if that chunks is lazly loaded / updated from neighboring chunks those entites stay

The expected behaivor should be not to create some entity every 0,5 seconds so you dont need to clean them up afterwards so even if the cleanup is broken right now you dont need it anymore

Ahh i have understood the problem after looking at your code

https://github.com/DrMango14/Create-The_Factory_Must_Grow/blob/1.21.1/src/main/java/com/drmangotea/tfmg/content/machinery/metallurgy/blast_furnace/BlastFurnaceHatchBlockEntity.java#L34

you try to drop the item even if there is none in the blast furnace inventory that creates a empty/(air) item stack. That atleast apperently can cause not to instantly despawn when some condition is met (prolly bcs that chunk was lazy at the time where it created 70k items)

Please add a check there if you can actually drop a item from the blast furnance inventory to not drop air every 0.5 seconds