I do not know the answer to your question and associated speculation for a cause regarding the add-ons. I am a casual user and do not have a detailed record of prior events. By most standards this is an abnormal occurrence, i.e. a bug. As such you should submit to the developers for reconciliation and to make the product better.
So far, there is a report of n=1 occurrences which may be just as attributable to a user issue and not a software issue. Should more people report this issue, then it would be looked at as a 'bug'. At that point, those people affected can write to Brad to report it as an issue or bug.
Really, if you do not know that you had just added the plug in or if it was carried over from the previous version of BeerSmith then it makes it difficult for anyone to try to help you out. As with any issue that comes up, the greater the information received surrounding the incident or occurrence, the easier it would be to track down the root cause.
Regarding the add-ons versus plug-ins, I pointed out a nomenclature issue and a suggestion for consistency and product improvement.
Finally, the typical Windows paradigm of multiple item selection would work as an example. Again, this is matter pointed out to made the product better. and satisfy some user's expectations.