My family member who has a vivosmart 5 ran into this issue when she first got the watch and wasn't in the habit of leaving the Garmin Connect app running in the background on her iphone. (She has a tendency to "clean up" all the background apps periodically by killing them all and was forgetting to leave Garmin Connect running when she did this.)
At some point she showed me the watch and all the info on it, time, steps, etc seemed to be "scrambled" with random incorrect values. After launching Connect on her phone and telling it to sync, everything on the watch thankfully returned to normal.
I don't know if it always happens, if it's random, or only happens in some unknown situation.
This seems to have been easy to fix, but it is a little concerning because watches like this could certainly go without synchronization for more than a day for various reasons and this really shouldn't cause them to exhibit this kind of non-determinstic behavior.
I have not tried to reproduce the problem again since it's not my watch and the owner probably doesn't want the inconvenience of trying to intentionally reproduce the bug.