Due to this Gap i was forced to use Create an invitation action in Power Automate, If you want to know about the difference between Send a Survey and Create an Invitation Click Here 

• We had raised a call and we got confirmation from them in July end Release there are many gapes which will be taken care of
  i.e Friendly name or alias name of Survey as currently Heading of survey and name of the survey are same
  Sharing of survey is mandatory if someone wants to view the Survey response
  And many small but useful gaps will be solved in next release
• Also for this Subject variable issue, they gave a workaround and thanks to them it working,
  Use {{Variablename}} in Subject manually to map the variable in the subject as below

  

• Also this will only work when used with power automate and not direct email.