I guess now we know the small reach of #Guppe groups is indeed a Guppe #bug or #WAI (working-as-intended).
I've noticed this since last year, after Guppe went down for almost a month(?); but I thought it was just me, or a temporary scaling issue with Guppe. But there's no denying it anymore, Guppe's reach is now very limited.
If similar software / platforms work fine (like #Friendica forums, #Chirp groups, #Kbin magazine, and #Lemmy communities) then the logical conclusion is this issue is on the side of Guppe groups.
Major communities still using Guppe groups should consider moving to Chirp, Friendica, Kbin, or Lemmy, if they want to reach #Fediverse network, until this is fixed.