That's intentional, although not ideal. The Comcode tag is the following:

[abbr title="full version"]contents[/abbr]

"contents", which exists on all tags, has the long text area, and is what shows up as the actual HTML. That's because in most cases / Comcode tags, it's the longest portion of the tag. "Abbr" is an exception since contents is actually the abbreviation, and you stick the long version in a title parameter (because in the HTML abbr tag, it's exactly that... < abbr title="long version" > abbreviation < /abbr >).

I cannot flip flop them because that goes against the convention of the contents being what shows up on the page (which is still true despite it being the shorter version). I can look into seeing if I can make the title field also long text.

Just seems odd that something like <abbr title="Domain Name System Security Extensions">DNSSEC</abbr> has the DNSSEC in a text area, and the abbr in a single line input when adding through the Comcode Tags interface, but your explaination makes sense.