¯\_(ツ)_/¯

Explanation:

The four normal DOM documents look like this (excluding < style > elements): 
1.---------------------------
index.html
...
  < portrait-frame >
    "¯\_(ツ)_/¯"
...

2.---------------------------
portrait-frame#shadowRoot
  < green-frame >
    < slot#portraitOne >
      "a picture"
    < blue-frame >
      < slot#portraitTwo name="label" >
        "of something"

3.---------------------------
green-frame#shadowRoot
  < slot#green >

4.---------------------------
blue-frame#shadowRoot
  < slot#blue >
Adding the "matroska Below is the flattened DOM _before_ "matroska" is added. 
...
  < portrait-frame >
    #shadowRoot
      < green-frame >
        #shadowRoot
          < slot#green >
            < slot#portraitOne >
              "¯\_(ツ)_/¯"
            < blue-frame >
              #shadowRoot
                < slot#blue >
                  < slot#portraitTwo >
                    "of something"
Important to note here is:
  1. blue-frame element is _slotted_ into green-frame.
  2. slot#portraitTwo is directly chained to slot#blue. But! slot#portraitTwo is NOT directly chained to slot#green.
  3. But since the host element blue-frame is put inside slot#green, that means that also the blue-frame#shadowRoot and the SlotMatroska it contains _also_ is contained within green-frame#shadowRoot and slot#green.
When "matroska" is added, this changes the flattened DOM and causes a slotchange event to be dispatched on slot#portraitTwo: 
...
< portrait-frame >                              _
  #shadowRoot                                   *3
    < green-frame >                             |
      #shadowRoot                               *2
        < slot#green >                          |
          < slot#portraitOne >
            "¯\_(ツ)_/¯"
          < blue-frame >                        |
            #shadowRoot                         *1
              < slot#blue >                     |
                < slot#portraitTwo > slotchange ^
                  < span >
                    "matroska"
And prints the following three logs: 
BLUE-FRAME     [slot#portraitTwo, slot#blue, document-fragment, blue-frame, slot#green, document-fragment, green-frame, document-fragment]
GREEN-FRAME    [slot#portraitTwo, slot#blue, document-fragment, blue-frame, slot#green, document-fragment, green-frame, document-fragment]
PORTRAIT-FRAME [slot#portraitTwo, slot#blue, document-fragment, blue-frame, slot#green, document-fragment, green-frame, document-fragment]
When this slotchange event bubbles upwards, it passes *all* three #shadowRoot and slot#blue, slot#green, and slot#portraitTwo, but not slot#portraitOne.