I've tested 3-6 channels opus files and they all break when processed.
command: rsgain custom -s i 30secsilence6channel.opus all opus modes break the file.

output:

[✔] Scanning '30secsilence6channel.opus'
[✘] Could not open input: End of file

binary diff between original and broken

-00000010: 5401 0000 0000 6e10 cf6a 011b 4f70 7573  T.....n..j..Opus
+00000010: 5401 0000 0000 957a b365 011b 4f70 7573  T......z.e..Opus

I eventually figured out that the error is caused by the zeroing of the header output (scan.cpp line 214) and that its specifically something to do with the crc as commenting out line 750 in tag.cpp stops the damage occurring.

The attached files are as named the 30seconds of silence file with 6 channels made in Audacity and the file after being broken.
testfiles.zip