Melting broken glass currently loses 1/3 of material (no slag, it just disappears). Melting scrap metal does not. Looks like an oversight in the broken glass recipe. Correct seems 24 broken in 24 molten out, resp. 48 in 48 out. Affects all furnaces.
See here:
https://discord.com/channels/803508556325584926/883590993063051275/1432299746785099866