"io.Copy inside CopyLoop io: read/write on closed pipe" in log of standalone proxy

Copied from the public bug-reporting pad.

My standalone snowflake proxy has been working well up to this point (at least as far as I can tell), but recently started to generate the error "io.Copy inside CopyLoop generated an error: io: read/write on closed pipe."

I got this message before occasionally, means, once within a few days, however since about 2 days this happens every single time straight after a connection has been established.

OS in use is Debian bullseye/sid, nothing about my setup changed.

Example output:

2020/10/12 17:01:05 starting
2020/10/12 17:01:16 NAT type: restricted
2020/10/12 17:05:38 sdp offer successfully received.
2020/10/12 17:05:38 Generating answer...
2020/10/12 17:05:41 OnDataChannel
2020/10/12 17:05:41 Connection successful.
2020/10/12 17:05:41 OnOpen channel
2020/10/12 17:05:42 connected to relay
2020/10/12 17:06:02 OnClose channel
2020/10/12 17:06:02 io.Copy inside CopyLoop generated an error: io: read/write on closed pipe
2020/10/12 17:06:02 datachannelHandler end

All I could come up with when searching for this error is #33211 (closed), which seems unrelated though, since there's no spike in CPU usage.

Can you point me in any direction on how to debug this (or is it actually fixable on my end?)?