This characteristic has been discussed with Mike Engelhardt, and this is a quote from Mike’s reply
QSPICE looks for the time to go backwards to detect one step from the next.
Don’t change TSTART if you want a flawless step recognition.
This characteristic is firstly discussed in this post, and solution is to prevent to modify Tstart in .tran
Step analysis on FRA analysis - QSPICE - Qorvo Tech Forum