Yes, I’ve seen that as well, and we got other similar reports too. The problem lies somewhere at the Core Location level, but we’ve had no luck getting to the bottom of it so far. I’m gathering some data to submit a bug report to Apple, maybe you could help me?
Could you try setting the adv interval to 100 ms (Apple’s official requirement for “pure/strict” iBeacon)? Could you try enabling the Basic Power Mode and disabling the Smart Power Mode? Does this happen with all of your beacons, or just some specific ones? If there are two iOS devices in range of the beacon, does the problem happen at the same time for both?