Future prototypes do not convert to Prototypes if all of their tech is available.
Designed a Future Prototype engine, Improved Thermal pulse, 0.7 Fuel use, no thermal reduction. Only needed to research 0.7 Fuel use, had 0.8 at the time. Engine prototype remained (FP) even when the 0.7x fuel use tech was acquired. Refresh tech and restarting the game had no effect.
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edit: more specific engine spec: Improved Thermal pulse, hull size 59, power 0.50, fuel rate 0.70, thermal 100
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Poked around the DB to see if I was still missing tech, TechSystem shows it requires BGTechs 24605, 25131, and 26091, and has Prototype=2. Race says I am game 33, race 186, and checking RaceTech shows I have techs 24605, 25131, and 26091, it seems I have all the tech it needs.
If this is not a bug, I'll make a suggestion over in suggestions... lol
Anyways, thanks for the super quick response times you've been giving.