The 2660 is still rather new from OpenWRT perspective.
So far three 2660 subvariants have popped up.
dir-2660 a2 (emergency recovery function requiring certain header bytes in flash factory file)
dir-2660 a1 (emergency recovery function ignoring header)
dir-2660 a1 variant 2 (a1 HW revision with emergency recovery code from a2)
Meaning: the vendor own recovery function which is squeezed in the boot partition of all 3 variants is not the same, the newer ones requiring certain headers bytes to be customized, while the oldest variant simply ignores those header bytes.
Since this recovery function in the device boot partition is the matter of choice to initially get OpenWRT onto the device, this is relevant for OpenWRT factory file creation.
D-Link has further added confusion, by assigning the same sales label to both variants of a1. Maybe variant 2 can be identified from the serial number, but so far no one has tried to pin down the sequence number as of when variant 2 rollout had started.
When this was found out, 21.02 was already in preparation, the relevant OpenWRT patch for the custom header bytes did not make it. So the 21.02.(0/1) release factory file for dir-2660 does not have the custom header bytes for the newer variants. The required patch although is contained in the snapshot for future release, e.g. 22.xx?