Can someone explain under what circumstances burn fees are applied to transactions. I'm using the same account, sending XTZ to the same destination. This morning I got no burn fees while testing, and now i've gotten 2, 5 minutes apart. These are very expensive, adding up to 0.5 XTZ + network fees, just for sending 2 XTZ.
Here are two operations not incurring a burn fee:
Followed by two incurring a burn fee of 0.257 each