rego-mode does not seems to respect TAB behavior set with `tab-always-indent`

[shackra]

I'm using Doom Emacs and installed this package through the means offered by it (migrating my old configuration to use Doom Emacs). I noticed that hitting TAB on any rego-mode file always insert a tab instead of just trying to indent the line despite having tab-always-indent set to t.

You can replicate most of my configuration with the following data provided by M-x doom/info:

SYSTEM  type       gnu/linux
        config     x86_64-pc-linux-gnu
        shell      /bin/zsh
        uname      Linux 5.9.14-arch1-1 #1 SMP PREEMPT Sat, 12 Dec 2020 14:37:12 +0000 x86_64
        path       (~/.emacs.d/bin/ ~/.emacs.d/bin/ ~/.zplug/bin ~/.cargo/bin ~/.cask/bin ~/.gem/ruby/2.4.0/bin ~/.gem/ruby/2.7.0/bin ~/.emacs.d/bin ~/.local/bin ~/opt/prey-node-client/bin ~/.deno/bin ~/.node_modules/bin ~/go/bin ~/go /usr/local/bin /usr/local/sbin /usr/bin /opt/cuda/bin /opt/cuda/integration/nsight-compute /opt/cuda/integration/nsight-systems /opt/flutter/bin /usr/lib/jvm/default/bin /usr/bin/site_perl /usr/bin/vendor_perl /usr/bin/core_perl /usr/lib/emacs/27.1/x86_64-pc-linux-gnu/)
EMACS   dir        ~/.emacs.d/
        version    27.1
        build      ago 28, 2020
        buildopts  --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib --localstatedir=/var --with-x-toolkit=gtk3 --with-xft --with-wide-int --with-modules --with-cairo --with-harfbuzz 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fno-plt' CPPFLAGS=-D_FORTIFY_SOURCE=2 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now
        features   XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND GPM DBUS GSETTINGS GLIB NOTIFY INOTIFY ACL GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS LIBSYSTEMD JSON PDUMPER LCMS2 GMP
        traits     (gui server-running envvar-file)
DOOM    dir        ~/.doom.d/
        version    2.0.9
        font       #<font-spec nil nil Fira Code nil nil nil nil nil 16 nil nil nil nil>
        theme      doom-gruvbox
        build      HEAD -> develop 026d96198 2021-01-31 04:34:46 -0500
        elc-files  1428
        modules    (:completion (company +childframe) (ivy +childframe +prescient +icons) :ui doom doom-dashboard doom-quit hl-todo hydra (ligatures +fira +extra) modeline nav-flash ophints (popup +defaults) treemacs unicode vc-gutter vi-tilde-fringe window-select workspaces :editor file-templates fold (format +onsave) (&nopath reformatter +onsave) lispy multiple-cursors snippets :emacs dired electric undo vc :term vterm :checkers syntax (spell +flyspell) grammar :tools (eval +overlay) lookup (lsp +peek) magit pdf :lang data (dart +flutter) emacs-lisp (gdscript +lsp) (go +lsp) json (javascript +lsp) (latex +lsp) markdown (org +dragndrop +pretty) (python +lsp) (rust +lsp) (sh +lsp) (web +lsp) yaml (&nopath rego) (&nopath pkgbuild) :email mu4e :config literate (default +bindings +smartparens +jorge))
        packages   ((hungry-delete :recipe (:host github :repo nflath/hungry-delete) :pin 0513152525c38519a5597db2d6495b56dd9cc3f0) (rego-mode :recipe (:host github :repo psibi/rego-mode) :pin be110e6cef5d34eef0529a8739c68e619cf15310 :disable (not (featurep! :lang rego))) (highlight-escape-sequences :recipe (:host github :repo dgutov/highlight-escape-sequences) :pin fae976568c04b6fe8a9f2d854c8fe23b357a6878) (pkgbuild-mode :recipe (:host github :repo juergenhoetzel/pkgbuild-mode) :pin 8a5e95c8514315cb40c47f1acdb68a4ace921497 :disable (not (featurep! :lang pkgbuild))) (reformatter :recipe (:host github :repo purcell/reformatter.el) :pin 45c0add95025f53ca644a6c8b9afa05b2da3c474))
        unpin      (n/a)
        elpa       (n/a)