https://www.kitguru.net/components/cooling/joao-silva/arctic-is-recalling-batches-of-mx-5-thermal-paste-due-to-anomalies/
 
According to Arctic’s statement, there are faulty batches of MX-5 thermal compound that made it out of production. The company explains that the affected compounds contain ‘anomalies in consistency and shelf life’, leading to increased oil separation within the paste and premature hardening. Nobody wants hardened thermal paste, with Arctic claiming this can cause ‘adverse effects and application difficulties.’
 
Arctic has already identified the affected batches and is recalling them from circulation to prevent anyone buying the dodgy MX-5. It seems that Arctic quality control wasn’t strict enough to avoid the shipping of such compounds. If you have bought an MX-5 thermal compound and believe it belongs to one of the affected batches, you can contact Artic via its customer support page. Affected products can be returned and will be replaced, without exception. 
 
Good to see Arctic responding to this problem fairly quickly.

Same here Steve.  I was going to get some when it first came out.  Thankfully I still have some MX-4 left...think I'll stick with that