In that such of a case, I would first check the servo wire routing, try to keep it away from "noisy" things like the motor or esc (and sometimes fans).

On the T4, I would think you have your receiver behind the servo with the esc behind the motor, so that should be ok.

After that, I've seen dirty receivers give weird signals, so maybe uncase the receiver and clean well with alcohol and a nylon brush, dry well and retry.

While that's drying, check that all your receiver plug wiring is in good shape for all components with no frayed wires or damaged plugs. Check the entire run of wire for pinched spots, especially on the ESC lead, but also your fan and transponder leads. I had a transponder missing laps a few months back that was also causing weird mid-corner steering loss because the wires were getting squeezed together in my routing when the chassis flexed. This needs special attention with the tendency these days to hide wiring anywhere possible for that clean pro look.

Way back in the boosted days, I had a first gen RS Pro that gave weird brownout and loss of servo function on the long straights that was eventually tracked down to where the wire entered the case and connected to the board, the wire (all three) was heavily pinched and wasn't keeping up on the current demand on acceleration. In that instance, I fixed it by opening up the hole in the case the wiring went through and replacing the lead with a heavier guage servo lead, which resolved my issues. I think that very same esc just finally gave up reading sensors under load, so it was quite a long term fix. There aren't many amps available from the BEC, so you don't really want to give up any needlessly.

I don't know what ESC you're using, but the BEC's can also degrade with age, further lowering input. Also verify your capacitor(s) are in good shape and well connected.

If all of that checks out without even a suggestion in your mind that something "isn't that bad", Tekin support is pretty good, and unfortunately RMA's are sometimes necessary across all brands. Seems to be slightly more prevalent in this pandemic world, unfortunately.