Hello again,

I did some more investigation, and was able to reproduce the issue.

However, I came up with a potentially more proper different fix: I think
the presence of the dotted pair with a non-nil cdr is the main problem,
since it means `erc-modified-channels-alist' does not fully adhere to
the documented structure in its docstring.  So I addressed the issue in
`erc-track-modified-channels' by wrapping up single faces in lists too,
ensuring that `erc-modified-channels-alist' never ends in a non-nil cdr.

Something along the lines of the following patch.  Thoughts?