daniel123 / marsch - I'm looking at the code carefully and there's just no way that the difficulty could be changing if the change-once-a-day check fails. However, as with the regular score changes, the score and difficulty could decrease twice in one day if there was an increase in between - if you answer the card incorrectly, then correctly, then incorrectly again, you'll see both of those incorrect answers reflected in the score / difficulty. And that's by design; if we didn't do it that way, but simply changed the score / difficulty once and refused to change it again regardless of what happened that day, you wouldn't be penalized at all for answering a card incorrectly as long as you'd answered it correctly within the past day, which doesn't really make sense.
To be honest, I'm not sure if the Automated scoring algorithm is such a good idea for Frequency-adjusted tests anyway - it's really designed more for Repetition-spaced ones, for frequency changes we'd probably be better off with something else that was much more gradual - perhaps something to consider in 2.1. The two new "scale" options improve matters somewhat but obviously there's a lot more we could do.
sinoreen - that's a longstanding problem, actually, but I'm nervous about fixing it because of the unintended consequences it might have on other E-C searches - it took us a long time to get that algorithm right and fixing an easy-to-work-around issue like this isn't worth causing severe problems in other searches in the process. Definitely something we'll fix eventually but it's probably too late to do anything about it in 2.0.1.