Found a bug that causes upgrading to fail.
1. Bought a Tier 1 Bubbly Coat, causing it to go in the stash.
2. Sold my current armor.
3. Bought the 2nd Tier 1 Bubbly Coat from the store.
This caused me to wear the newly bought Coat, but it didn't upgrade merge with the T1 Coat already in my stash. If I try and click the Coat in my stash. The game goes into a pseudo infinite loop of playing the swap item sound. But other than that nothing happens.