Thanks for the additional information. The second video makes it easier for me to interpret.
It is behaving in the same way as I have experienced (if your tablet has an unreliable or poor internal GPS receiver it is not unusual for it to change location and show low accuracy when it initialises - ODK Collect is grabbing the first data available). Some older devices I have used could take 20 seconds to get a good fix (even from warm-start), so I became accustomed to keeping their GPS signal 'live' with a secondary app running a track...
If the connection to the Garmin drops temporarily, your device will probably fire up the internal GPS - and if it is struggling to get a fix, this is the behaviour you would 'expect'. And then when the Garmin reconnects, it will jump back to your actual location.
My suggestion is to use another 'navigation' app to test if the same thing happens - not to ask whether you are using more than one app at once. If Bluetooth GPS app has logging available you could go through that. to see if there are drop-outs - it may not show up, hence suggesting the visual test in another app.
I'm not yet convinced it is an ODK Collect specific issue. If you can confirm whether the same behaviour happens with other apps, using the same device(s), I think that would help point the finger more clearly.
As an aside, the Garmin device appears to have accuracy of approx 6m, which is no obvious advantage on an internal GPS on an Android device - you probably have other reasons for using it and therefore need mock-location?