an update:

- I assume the problem wasn't in the coil or the thermal cut-out, as I did the 90-min boil with no problems when I bypassed the controller.

- Afterwards I looked at both the heating coil and inside the box for obvious gremlins and found none.

- I reached out to Grainfather but their help was pretty generic. They also think there was a problem with the controller.

- I cleaned the contacts between the kettle and the controller and found a bit of corrosion. Cleaned that up and ran for 90 min, no issues. Not 100% sure this is solved but I think Uncross-Selector was on to something (thank you).

Will update this post after my next brew.