Flow's visual editor doesn't know about extensions tags. Using an extension tag in a wikitext post, then attempting to switch to visual editing, causes all kind of funny business.

For example, consider the following silly but well-formed wikitext:

<charinsert>derp</charinsert>

<indicator name="herp">derp</indicator>

<templatedata>{"params":{"derp":{}}}</templatedata>

<timeline>
ImageSize  = width:200 height:25
PlotArea   = left:0 right:0 bottom:0 top:0
TimeAxis   = orientation:horizontal
Period     = from:0 till:100
PlotData   =
  from:0 till:50 color:red text:herp
  from:50 till:100 color:green text:derp
</timeline>

The normal VisualEditor doesn't know how to handle any of these tags, but it knows that it can just show a generic editor: https://en.wikipedia.org/w/index.php?title=User:Matma_Rex/sandbox&oldid=768103527

Flow's VisualEditor gets confused and displays some of the content as if it was editable, but most changes you can make are ignored if you try to save (or switch back to wikitext): https://www.mediawiki.org/wiki/Topic:Tlywf7o3ljghil98