New to power-based training: 3 months in, FTP dropping/fluctuating

Good point, I will do that. As I said, I haven’t been training for about 10 years, so my understanding of how certain workouts should feel and how I react on them is very limited.
I didn’t feel like going overly hard. Recovery was fine. Workouts are so short that (max 1.5h mostly) that the next day I don’t feel that anymore, much like how my winter spinning used to be when I was still in competition: very intensive, but quick recovery.
I just eat normally. I don’t really change anything based on my training. For tough workouts I usually prepare a bottle with an isotonic drink in addition to water.