Early Ice Crystal Problems?
I worked Aeronaves de Mexico's Britannia's at JFK way back when. They were Model 302 with only four tanks and so had to fly fairly high to make Mexico City - New York. They had continual problems over the Gulf of Mexico with "Bump Stalls" caused by ice crystal ingestion. I seem to remember they blamed alto-cirrus clouds as the source. An "intrascope" (early borescope) inspection was required and I saw more than a few twisted and bent compressor blades and on occasion what we called "corn-cobbing". I leave that to your imagination.
Realize the Proteus was a first generation engine with reverse flow internally. It only had glow-plugs instead of continuous ignition. Was this icing condition related to the present generation ice-crystal icing problems here?