If you feel it is a bug with reainsert , write a bug report if have time.
I only ever tried one instance of reainsert and got different ping values every time for a straight round trip pass so something was up with it.i just remember it not being reliable like on cubase but cant remember if i checked again on reaper v5.
Apparently turning off anticipative fx processing for that track helps but does not solve. Dont have access to reaper at moment as laptop died but maybe other settings affecting it too ?.
Remember you can always just route return track to new track and nudge back in time manually after recording, but thats a pita to do.
Here are reainsert related bug reports :
https://forum.cockos.com/search.php?searchid=14363911
Cheers