Steps to reproduce the issue

Joomla 3 or Joomla 4 - both do this

Go to Admin -> Users -> Mass Mail Users

Compose an email

Select a group that has no users in it

Click Send email - note correct error message saying group has no users

Give up and click Cancel, can't be arsed to email anyone anyway...

Do something else for a while, Grab a coffee, edit some content, look out the window...

Go to Admin -> Users -> Mass Mail Users

Expected result

Blank form ready for some new mail I want to compose.

Actual result

The contents of the last form submission (The composed email) which failed to send some time ago.