Last week we had Update 1.2 installed in our CRM 2015 Online instances. After that we have started getting following issue.
In alert we were showing Label of the field using Xrm.Page.getControl(arg).getLabel(), and after update it is now showing Lablel as well as description of that field.
Waiting for the fix, till now removing the description seems like the only solution (temporary one)
Hope it helps