This is also happening for me, when using the Send an Email from a Shared Mailbox (V2) action. It lets me create the action, and save the flow, but when I try to then edit the action, the entire window freezes and the only option is to close the browser tab.
It only seems to happen with some flows - I have two instances of this action in 2 flows, both with complex HTML in the 'body' field and Dynamic Content in other various fields.
Is there any way this can be resolved?