Summary: Routine ground unit class design resulted in a "Race-designed Engine" research topic (in Power and Propulsion) with an unusual RP requirement which when researched did not create any component nor ground unit.
I finally researched Advanced Genetic Enhancement and decided to create some top-of-the-line boarding troops. I selected Infantry, CAP, Heavy Powered Infantry Armor, Boarding Combat, and Advanced Genetic Enhancement, which results in a research cost of 273 (Image 1). After clicking "create" I went to the economics->research screen to research my new ground unit but the tech wasn't there. Certain that I had indeed clicked "create"-but without searching too hard-I decided to design and research the HQ for these elite boarders which occurred normally, while waiting to see if the research topic for the CAP troops magically turned up.
About 3 months [and a few IRL hours plus a save/quit/load] later, as I was cleaning out a bunch of test prototype engines by making them into research prototypes and then deleting their project entries, I noted the missing ground unit class research topic in Power and Propulsion which had the correct name but an RP cost of 501 which does not correspond to the cost of 273 noted above (Image 2). I SM-researched it but there was no result: no engine in the class design nor technology report windows, and no ground unit class available in the ground unit formation template screen. DB attached in case it helps.
I can't replicate it, designing the same class again achieves the desired normal result. I have seen this problem before so I suspect (hope?) others have as well.