Im playing greedycraft, and added universal tweaks + lootr and its dependencies. (with the mods that UniTweaks patches removed from the game)
upon summoning any minecart with chest, no matter the loot table, it has no loot

Tried summoning multiple loot tables. Chests work fine, just not minecarts with chest

Unfortunately, the 1.12 version of Lootr currently has no maintainer, and I'm not able to support it myself as I haven't developed for 1.12 in a number of years.

Ahh dang, thats unfortunate. :(
Im sorry to hear about that.
I could try commissioning someone to fix and PR the update.