Little update.
Since my last post in this thread, I have had 4 pairs of Meelectronics Sport-Fi S6 replaced by warranty, so I no longer can recommend them. Bonus for Meelectronics honoring the warranty each time though.
I replaced my S6 with JBL Synchros Reflect BT, which I now recommend as a good sport IEM (or if you prefer wired version, then just JBL Synchros Reflect). I couldn't be happier with my choice. These JBL earphones stay securely in my ears during any workouts and on top of that they sound incredible. Bluetooth is working fine, no cutouts, hisses, or anything in the sound.