This should not be a browser issue as Easel does not require internet access while passing the generated gcode to the controller.
You mentioned that it happened on a “a complicated 3D carve” and the “last of 6 boards to be carved and two of them had an intricate design that took a couple of hours each to carve.”
Typically, when the generated toolpaths in Easel appear normal, the issue will lie with the machine itself. If there were a communication issue between the computer and the controller, this typically results in the carve ending sooner than expected and prior to completion. Since your machine is “going rogue” in the middle of longer complicated carves and when being operated for an extended period of time, this makes me think that your machine is loosing steps during the carve possibly due to the stepper drivers overheating. Have you checked the potentiometers for the drivers? Does your machine ever stutter while moving along an axis? It “could” be a loose connection, but this is normally observed as stuttering movement all the time, even with no load. You should be able to see/hear either of these issues, but if it is thermal overload during a carve, you might miss it due to the movements/noise of the actual machine motion/carving. I think it is worth investigating.
These might be of interest to you…
Instructions for setting the current limit potentiometers on the X-Controller…
Other videos…
These are older, but still relevant for understanding the issue…
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Brandon R. Parker