On Connect, we redirect the root of an API to /__swagger__/, plumber then redirects that to /__docs__/, however with e339c15#diff-29631ab96e5f2e52db5bd0071efcdbe7cea6750ef5c2932a3c213bf2507ff6a7L275-R279 that second redirect would now change the URL and the actual redirect would end up at the connect-server-level docs.
We have added a change to Connect that directs to /__docs__/ directly which sidesteps this issue which will be in the next release (2026.01.0), but I want to raise this issue here since it might also trip up others / might be worth revisiting in plumber itself.