Not sure if this should go with goguma or soju, but soju's update introduced this bug.
In the past, soju has forced CASEMAPPING=ascii (see also #149), making nicks that should contain \ (backslash) appear with | (pipe).
In soju 0.5.0, there were some changes to casemapping that seem to confuse Goguma. Push notifications and direct message buffers are displayed with the casemapping they should have (rfc1459 mapping, showing a backslash), but in channel conversations, ascii mapping and pipes are shown. In direct message buffers, messages from someone with a backslash/pipe in their nick do not show up (I'm only able to read messages that appear as notifications).
The above behaviour only happens on Libera. On OFTC, Goguma creates a buffer with the correct casemapping (backslash), but incoming messages are shown in another buffer, with pipes.