I have a Garmin Fenix 5S and I use the 'resume later' function a lot because I often do more than one activity during the day and don't want to create separate records for each one.
A lot of the time, when I resume my activity I find that Garrmin has added an extra mile or so to my distance. So, if I've done a 2-mile run to stretch my legs in the morning I'll come back at lunchtime and it says I've done three miles.
It's a bit frustrating so if anyone else has solved this problem, your tips will be welcome!