I've noticed this phenomenon with anything creative, stepping back is not always enough, you need to step away for long enough to have some perspective - Then if your opinion of your own work changes it's difficult to know if it's due to re-framing or because you've been exposed to other tastes in the interim... it's all a little maddening. I try to differentiate, it's important to know if your preference is fleeting, due to exposure to something of novelty, or something lasting.
Similarly with UI design, even within the "flat" era I see things oscillating back and forth, over-rounded buttons, over-squared buttons, over-rounded buttons, over-squared buttons, back and forth, Google does this constantly as if they can't make up their mind... is it just fashion? is the new black completely dependent on the previous black?
After a while It makes you think very little in life is objective, we are all part of this interwoven thread of culture giving context to all.