Spektrum released the new public beta firmware 2.43.2 for remote receivers (SPM4650 / SPM4651T) that addresses the following receivers issue:
On initial RF connect and during re-connect after a hold, there were a small number of frames where Preset failsafe throttle data was being sent marked as normal channel data instead of failsafe. This made it impossible for FBLs and FCs to ignore, and resulted in throttle dropping to 0 momentarily. The soft-start function of the ESC then resulted in an even longer time with no throttle.
If you want to avoid unwanted ESC soft start activations while flying, UPDATE the FW of your 4650 / 4651T receivers!