I’m also seeing the same on my TS22. There’s about a 20% drop in lumens once the battery is below around 3.85V
However, I found a workaround: if you crack the tail cap, while the light is on, then you get back to full 100%. I also verified this using a current clamp meter.
Similar to my Convoy S21E, braking the circuit (crack the tail cap) while the light is on will allow the light to come back on in the same mode – even turbo for the TS22 – once you complete the circuit (tighten the tail cap). I assume this is to support a forward clicky tail switch.