After 3 days of testing it seems that MT finally fixed this in 6.48.1 as stated in the patch notes.
No flaps now for the mentioned 3 days even under heavy load conditions.
Here's hoping it stays that way.
Its seems this DMZ actually does what it saysYou mean there is another router (provide by ISP?) before the mikrotik? If so see if that router can set to "bridge mode"
sometime DMZ won't solve double nat problem