So 2 months later, using the Sonicgear TWS 5 Pro earbuds...
I don't use them every day, and I keep them in the box when not in use. They haven't been exposed to any extreme temperatures, and I didn't drop them once. I even turn off my phone's bluetooth when I'm not using the earbuds (in order to save the phone battery mostly).
Today the right earbud just ran out of battery after some 30 minutes of use. The left one was fine and kept going (they can be used independently of each other).
Then I put the earbuds in the box for some 20 minutes, and the left one charged fully. The right one didn't. Then I took the right one out of the case and paired with my phone, and it said 100% battery. But then I put on some music and literally every minute it would drop 10% battery. WTF

Now I've had them both in the box for about 2 hours. The left one is fully charged. The right one though, it's like it won't fully charge. There's a red light that should turn off when the earbud is fully charged. It just won't turn off. So I plugged the box to the lap top to recharge the box, and I'll leave the earbuds in like that until tomorrow. A guy on reddit mentioned he tried charging the case with the earbuds in when he had the same problem. It didn't help him any, but it's worth a shot.
I fully expect the earbuds to not last forever, but I bought them like 2 months ago, and that's too little time for them to be having problems, especially considering I often don't use them for days at a time, and nothing has happened to them to cause this problem.
If the problem persists, I'll have to claim my warranty. I should have a 2 year warranty. But you know how it goes with warranties - you're covered for everything except the problem you're actually having.
Meh...