BLE Spec 5.1 Vol 6, Part B Section 4.4.2 Advertising State now says:

“At most one advertising PDU shall be sent on each used primary advertising physical

channel index in an advertising event. Unless specified otherwise, the primary

advertising physical channel indices may be utilized in any order.”

Under certain conditions, the BLE Stack may repeat the advertisement on a channel previously used; for instance, 39 -> 38 -> 39, on a single advertisement event.