This is an option you have if you have UEM where you can set a number of 'conversion goals' which can be things such as hitting a certain page, logging in, or making a purchase via measures for page views or even methods that are called. Then each UEM visit will be evaluated to determine if this conversion goal was met (i.e the user logged in or made a purchase). Thus the conversion rate will just be percentage of visits that were converted - by dividing the converted visits by the total visits in that period.
10 users visit the site and 5 make a purchase (/checkout page viewed) then you would have a conversion rate of 50%.
There will be an overall conversion rate as well as an indvidual conversion rate for each of your defined conversion goals.
This page has some more info on user experience settings including conversion goals:
Nothing that I am aware of and I would be very surprised if that was the case. How have you determined that the data is incorrect and in what way is it incorrect? If you do think something like that is wrong after an update you should open a support case.
What version did you upgrade from?