I have had the same problem, for around the same time! Except, I think, in reverse-- recurring events have disappeared from icloud.com but remained on my iphone.
Scared the living daylights out of me!! as, once I have input stuff in iCloud, it is wiped from my brain and I become utterly dependent, including for work purposes, on that 'ping'.
I have found that, if I switch view from my normal view (Week) to Month, and then switch back, my recurring events reappear. BUT-- because I am so dependent on iCloud, and for very important things, I remain very worried! If their system can do this and they can't seem to work out why, what else might it do?
I mean-- I know I'm using it as a free service; but stuff like this can still damage Apple's reputation. I hope they do something about it-- and flag up what has been done in some way the information is easy to find.