Unicode bugs?? -- font or emacs?

[Rusi]

[This is more of a meta-question than a question]

If some misbehavior is found (seemingly in emacs) surrounding exotic unicode 
chars
how does one debug whether its an emacs thing or a font thing?

⟮ ⟯ ⟪ ⟫ 𝗮𝗯𝗰𝗱𝗲𝗳𝗴𝗵𝗶𝗷𝗸𝗹𝗺𝗻𝗼𝗽𝗾𝗿𝘀𝘁𝘂𝘃𝘄𝘅𝘆𝘇 
The above chars were being surrounded by many spurious lines

I thought that this was emacs trouble
Changing font seemed to change/reduce trouble -- so font not emacs??

However I also see that some editing actions around these chars makes the 
problem
disappear (sometimes; not sure when)

So now what?