When I looked at their Barbarian’s armoury page and scrolled to the bottom, it says…
Last updated on Sep 10, 2021 10:11 AM PDT
Anyway, their build is all over the place. Nominally, it’s a Whirlwind/Rend build, but the Barb only has 4/6 set pieces equipped, doesn’t have Bul Kathos weapons (to maintain full fury), isn’t using Ambo’s Pride, Lamentation or Mortick’s Brace (either equipped or cubed), so my best guess is that they’re WW’ing, running out of fury, being unable to continue WW’ing and having to use primary attacks to build up enough to start WW’ing again.
They need to read these…
https://maxroll.gg/guides/waste-set-ww-rend-barbarian-guide
Short version - not a bug.