The Beltlayer Underground Belts icons (rectangular edges) do not show in the Crafting Menu. I have researched Underground Belts, so the vanilla underground belts icons do show. I've not played Factorio for over a year. When I played back then, Beltlayer worked.

sidebar: Pipelayer icons do show in Crafting, and can be crafted. When I place them, they work as advertised.

Can you upload the save where you are experiencing this issue?

i think if you install the mod after you've done the research, it doesn't show the underground belt in the crafting menu...or maybe it only shows up correctly in sand-box mode.

https://i.imgur.com/S3Fyzyo.png

https://www.dropbox.com/s/ei5qsrbl1ildukk/JoeyDeathRichLaxed.zip?dl=0 save file

I also have alot of other mods installed, but this mod works in sandbox mode so i don't know that it's them. and pipelayer also works.

If I use console to undo the research and research it again - it works. That disables achievements though.