Ok them being not charged wouldn’t cause the symptoms, so suspect the act of putting them back in the case you actually put them properly in the case. It’s one of them not properly in the case would cause the symptoms. If you used both buds and then thought you had put one in the case, but you actually hadn’t then it’s still connected to the phone then when you distant from the case then you’re actually putting the distance between the two active buds, so causing distance from case to cause issues.
For the Air 2 the most common issue of not actually putting in the case are for new buds you didn’t remove the plastic, then it’s because you swapped the ear tips for larger and they push back up in the case so they don’t press into the charging pins at the bottom, and eventually it’s debris dirt grease on the pins.
Over the life from brand new noob error through to eventually need to clean, these are the common issues with LA2.