My testing minimal init.vim
The part of the mapping that is relevant:
require('astronauta.keymap')
vim.keymap.inoremap{'<Tab>', require("completion").smart_tab, buffer=true, silent=true}
vim.keymap.inoremap{'<S-Tab>', require("completion").smart_s_tab, buffer=true, silent=true}
How to reproduce
Compile the upstream at this commit. Use tab completion.
Expected behaviour
Completion popup to show up, but it doesn't.
I can confirm that completion works at this commit.