I have an organizational account through the ArcGIS Home Use program. At the 2017 UC, I found myself with a credit balance of zero. While still at the UC on 7/13/2017, I purchased an additional block of 1,000 credits.
Fast forward to this past week. On Thursday (7/12/2018), I had somewhere in the neighborhood of 740 credits. On Saturday, I log into my account and see that my balance is now 93 credits. I've done nothing with my account in the intermediate days (no adding layers, using premium content, etc) so I'm at a loss to explain the drastic reduction in credits other than ESRI removed the remaining block of unused credits 1 year from their initial purchase.
But that doesn't make sense either because on the ArcGIS website it states that additional credit bundles are good for 24 months. So are additional credit bundles purchased for a Home Use organizational account treated differently?
(I haven't done it yet but I will be contacting customer service later today about this)
P.S. Shoutout to this ArcGIS Idea about rollover credits.