I've been able to narrow this down to the following example:
var MN =
version : '071100010002',
nonIE : document.attachEvent,
otherProperty : "Another"
When running ctags on this I get tags for version and nonIE but not otherProperty. If I remove the '.' then I get tags for all three.
On a related note for some reason I'm not getting a tag for 'MN'.
This is all with today's code.