Disappearing subscribed calendars

I'm having a problem with iCloud and Mountain Lion's Calendar app.

My operating system is up to date. I'm running Mountain Lion, and the App Store doesn't show any available updates. 10.8.2.

I have a calendar on OmniFocus's sync server located at webcal://sync.omnigroup.com/xxx/OmniFocus-Reminders.ics (where xxx is my username for their sync server). I add it as a *subscribed* calendar in the Calendar app, and tell it I want it to sync with iCloud. This works OK; I see it on the Mac and on my iPhone and iPad. If I delete it, it goes away in all three places as I would expect. I can add it again, and it shows up everywhere. So far so good. Apparently iCloud just syncs the *web address* for the calendar, and then each device goes and gets it from there.

That's all cool.

However, I'm experiencing an issue. The calendar keeps disappearing from the Mac's Calendar app. It isn't there. Then I do a manual refresh (commnad-R) and it might show up for a second and then disappear. After maybe 4 or 6 refreshes, it comes back and stays there until the next manual or automatic refresh. It's gone again. Since it's a subscribed calendar, the whole calendar just goes away. The calendar name just disappears from the calendar list on the left of the Calendar app. (All the events in the calendar go away too, but they come back if I can get the calendar name to show up again by manually refreshing.)

This affects two different Macs, both linked to the same iCloud account. Behavior is identical on both computers. It does *NOT* impact the iOS devices at all. The calendar stays there on iOS devices, doing what it should. It's rock steady.

Since iCloud is supposed to "just work," I can't see what iCloud is doing. If I log into my iCloud account on the web, by design the subscribed calendars do *NOT* show up.

I've deleted the subscribed calendar completely. I've also gone into System Preferences on both Macs and turned iCloud for Calendars and Reminders off and on again. This does not help.

Has anyone else experienced this kind of behavior with a subscribed calendar?

I should also mention that I also haev a subscription to a TripIt calendar (with an address that also begins with webcal://) and it has *NO* problems at all. I called and talked with the excellent folks at the Omni Group. They are sympathetic to my plight, and are really great to work with, but don't seem to be able to help me out. They haven't seen anything similar wtih other users. Since their server is responding properly and is serving the calendar just fine to my iOS devices, it seems that it is a bug either with iCloud's implementation or with the Mac's calendar app.