Bluetooth OTA fails: iOS DFU Library complains that Image 0 (slot 1) is already pending

[responsible_duck]

We are suddenly getting lots of cases where devices aren't able to perform Bluetooth OTA via our iOS app. 

Primarily this is what we are seeing is this: `error=There is no free slot to place the image`.

First we start the update:

This is what the image list response right before this looks like:

dfu: Image List response:
  Header:
    version   : "1"
    op        : 1
    flags     : 0
    length    : 248
    group     : 1
    seqNum    : 60
    commandId : 0

  Payload:
    images:
      - bootable  : true
        hash      : 0xAA00383B0829FA8BA63CC09BD126D7CBEF866D9A8DD8C9BFD176EE13A8FDAAA6
        active    : true
        permanent : false
        slot      : 0
        version   : "0.0.101"
        confirmed : true
        pending   : false

      - version   : "0.0.103"
        hash      : 0xA576F7F8B294DA3A26979D8846E0D2CCE6454370F8A57B0E9C7053EDD5526884
        active    : false
        slot      : 1
        pending   : true
        permanent : false
        bootable  : true
        confirmed : false

    splitStatus : 0

The library throws this error:

We also see `dfu: Image 0 (slot 1) is already pending.`

but proceeds to upload image. Device then restarts upon upload completion but seems to restart on the old firmware, showing the same image list. A further update attempt throws this error:

dfu: There is no free slot to place the image

and stops the update.

We use a `testOnly` strategy (not testAndConfirm) from the library so that we confirm the image in the app. This was working quite fun for a few months until now.  

We see this commit which attempted to fix a similar issue but it seems like it did not resolve this specific case. Also saw this devzone ticket but we didn't make any changes to pm_static.yml and how the secondary slot is allocated. 

We also tried force testing the image stuck in secondary slot using `mcumgr` with no luck.