There have been multiple reports in the discord server of pipes not putting items into chests when they should, or outputting items from hoppers. The solution for this in all the cases was a game restart (both on SSP and SMP).

After some testing, it happens that if you connect a pipe to a hopper and immediately go down (image), the pipe will output the items from the hopper and put it back to it. (this is one of the described problems), restarting the world does not fix it, you need a full game restart.

1.20.1