why not upgrade it to RC11 ?
Does it specifically fix this issue?
I don't know if it fixes your issue or not, I suggest you check the change log. Since you're running on out of date beta code it would seam pointless trying to work out what's wrong with it. Upgrade, if the issue continues then contact MikroTik.
Thats the general tactic employed with things like Cisco gear too. Weird bugs? Upgrade IOS first, then start trying to work out whats causing it, only if it persists.