I bumped up against this problem a short while ago on a project. I eventually concluded that the problem occurs because the default constraint is given an arbitrarily assigned name on the subscriber and because that name is different to the name assigned by the publisher when you add the column, it does not know what to delete at the subscriber when you drop the column. I concluded as did you: drop the constraint at the subscriber before you drop the column at the publisher using the using the repl stored procs or EM.