forestry_1.12.2-5.8.2.366, the version in the latest dw20 ( @FTBTeam )
an illustrative video

Does this recipe work?

No, not even if I warm it up with another recipe first, I just can't click the cast into it.

both casts I'm trying have a metadata of zero, I can't insert it (into that slot) via side or top, I can't JEI [+] it into the slot.

I tried as well in the DW20 pack and it didn't work. So it seems like there is a conflict/recipe change somewhere that breaks this.

The fix wasn't explained but here is the same issue: AllTheMods/ATM-3#752

this issue hasn't been solved yet? I'm playing on FTB Revelation which has forestry 8.1 I think, and the wax cast doesn't work

shown with enchantment, bug existed without enchantment, opted to see if it changed anything (no)

Please try without baubles, forgot to comment on this issue too

that's gonna be tough, can't disable it without also disabling thaumcraft, astral sorcery and another dependency that i forgot

Sure, but you could create a test world and it might prove that at least it causes the conflict.

I used FTBPresentsDirewolf20112-2.5.0-1.12.2 and disabled:

astralsorcery-1.12.2-1.10.11.jar.disabled
Baubles-1.12-1.5.2.jar.disabled
Botania r1.10-360.jar.disabled
Thaumcraft-1.12.2-6.1.BETA26.jar.disabled
ThaumicJEI-1.12.2-1.5.5-23.jar.disabled

Then the wax cast worked fine. So there is a cross mod conflict with one of these (baubles almost certainly).

I haven't looked far enough to work out whether this is something weird that forestry is doing but given lots of other mods seem to work fine I would say it's quite likely that baubles is causing this issue.

Ok baubles had apparently a ton of dependencies, after disabling quite a lot of mods I got to make the wax cast work, here's the complete list:

Modpack: FTB Revelation (Modified)
Mods:
ActuallyBaubles-1.12-1.1.jar.disabled
astralsorcery-1.12.2-1.10.20.jar.disabled
Baubles-1.12-1.5.2.jar.disabled
Botania r1.10-361.jar.disabled
deepresonance-1.12-1.8.0.jar.disabled
EmbersRekindled-1.12.jar.disabled
EnderIO-1.12.2-5.1.51.jar.disabled
Exchangers-1.12.2-2.8.jar.disabled
IntegrationForegoing-1.12.2-1.9.jar.disabled
mcjtylib-1.12-3.5.4.jar.disabled
nice-1.12-0.4.0.jar.disabled.
notenoughwands-1.12-1.8.1.jar.disabled
rftools-1.12-7.72.jar.disabled
rftoolsctrl-1.12-2.0.2.jar.disabled
rftoolspower-1.12-1.2.0.jar.disabled
secretroomsmod-1.12.2-5.6.4.jar.disabled
SimplyJetpacks2-1.12.2-2.2.13.66.jar.disabled
Soot-1.5.jar.disabled
haumcraft-1.12.2-6.1.BETA26.jar.disabled
ThaumicInventoryScanning_1.12.2-2.0.10.jar.disabled
ThaumicJEI-1.12.2-1.6.0-27.jar.disabled
theoneprobe-1.12-1.4.28.jar.disabled
topaddons-1.12.2-1.11.0.jar.disabled
twilightforest-1.12.2-3.9.984-universal.jar.disabled
WirelessCraftingTerminal-1.12.2-3.12.93.jar.disabled
xnet-1.12-1.8.2.jar.disabled

Thanks for investigating! I've narrowed this down a bit further and I suspect that Astral Sorcery is the culprit here. I've opened an issue on their tracker: HellFirePvP/AstralSorcery#1191