Positional errors occur for a number of reasons. One is poor satellite view due to terrain and/or foliage, or the device using a reflected signal(s). Another issue is satellites coming into and passing out of view. It takes a little time to update the more accurate position parameters for the new satellite, but the device may use approximate use the approximate parameters until the update is complete. If satellite coverage is poor to start with, this could lower the position solution from 3D to 2D. Also, view loss of the WAAS satellite can add to the error.
The only thing you can do is duplicate your track and delete the obviously errant points. The eraser tool should work fine.
As to prevention, there is not much you can do. It is best to carry the device in the open with the antenna pointed straight up in locations where satellite visibility may be poor. Another alternative is an external antenna to improve signal detection if your device will accept one and one can still be found. To be honest, I'm not sure how much of an improvement you will see as the newer devices are quite good. Also, not blocking the southern horizon with your body can help, assuming that the south isn't already blocked by terrain.
I did this hike with Fenix 5 and an Edge 820. Both completed the hike with no discernable error. I use the 64s for winter activities because I can operate it with gloves on. I wear it on my chest up high with the antenna vertical. A
A chest pocket is not a good location. I generally carry mine in my hand cupping the bottom of the device or I'll hold in a side jacket pocket with the antenna sticking out and tilted slightly away from my body. I switch to the side away from vertical obstructions as needed.
The recommendation of maintaining a southern view (northern view in the southern hemisphere) only becomes important the further you are from the equator. Satellite orbits do not pass over the poles and I think they are limited to +/- 80 degrees in latitude. This creates a void area area towards the pole, so blocking satellites toward the equator can be a real issue. I would not think it would be much of an issue where you were.
I don't own a 64, so I can't tell you if the Glonass satellites are enabled by default, so you may want to check your device set-up - enabling them would make large position errors less likely,
