Using abbrev-prefix-mark to add a suffix starts a new line

[dbdkmezz]

This is odd. I discovered that I could use abbrev-prefix-mark `M-' to
add prefixes and suffixes to abbreviations yesterday, and it all
worked fine (I think!). But now when I use to add suffixes it adds the
suffix fine, but then moves the pointer down to the next line (if
there is a lower line to go to). I know there's nothing in the
documentation about using this command for suffixes (it only mentions
prefixes) but it makes sense, and it does at least half work.

Anyone have any ideas about what might be going on, or how I can fix
this?

Thanks

Paul