Looking for information on Server/Data Center? MultiExcerpt and Draw.io in Data Center and Server
If you use the "simple viewer" option on your Draw.io diagram to restrict editing and place it in a MultiExcerpt and include that in a MultiExcerpt Include it will not properly render
To properly display the draw.io diagram in MultiExcerpt Include, you must use a MultiExcerpt Fast Include (see Difference Between Multiexcerpt and Multiexcerpt Fast).
You can use either a Block or Inline fast include.
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More information on Draw.io documentation on "simple viewer"
This behaviour is the same in the new/fabric and Legacy page editor.