Loosing connection between MacBook and external monitor

<< The connection is made via a Thunderbolt cable that came with the monitor (so I assume that it is made to a standard that works with 6k) providing power to the MacBook. >>

Cables "shipped in the box" are NOTORIOUS for being 'lowest-bidder' cables -- only good enough to keep you from returning the device on the first day.

To run a high-end Mac display on a ThunderBolt cable, that cable must be certified (as evidenced by the ThunderBolt symbol marked on each end), and limited to a maximum of 0.8 meters for Apple cables, and 0.5 meters for brand-X cables.

It's not exactly low quality that is the issue.

it is the ability to run, ERROR-FREE at speeds of about 26 Gigabits/sec, without fail.

To run at those speeds without errors from signal attenuation and distortion that causes data errors, the cable must be shown to be capable of running at those data rates, error-free. Rather than Users testing each cable, we simply use cables that meet a high standard.

if you set the resolution down to 4K at 60 Hz, the lower-spec cable should work fine, because the data rate used is substantially slower (and therefore far less likely to have data errors at that speed).

Remember that the standard debugging step of substituting a different cable is a hedge against BROKEN cables. In this case we need High Spec cable, and nothing less will suffice.