AirPods only play one at a time

My friend came across a pair of AirPods that were in a case with the light on the inside, but the case was assembled in Vietnam. I can't identify the case number. However, they lost the right one, and I decided to buy them a replacement. I assumed that it was first generation due to the case, however they were second generation AirPods in a supposed first generation case. I then switched out the replacement for a second generation right airpod (A2032). Looking at the model number of the Airpods, they identify as A2031, so I thought it would work. The case paired the two together and they connect as a set. The audio works perfectly fine on both Airpods, and the charging status always works on the iPhone. However, only one Airpod will play sound at a time, and it's inconsistent as to which one plays. I have contacted Apple support, but they don't know what to do. I wonder if I should buy a replacement 2nd Generation case, as the mismatched case may cause synchronization issues between the two buds. Anyone know what to do?