Just like "send email" action processes where an option in advanced section for "From (Send as)" is available, the same should apply to Approval process as well and it shouldn't go from a generic email such as firstname.lastname@example.org.
In my scenario;
Step 1 (send email): User fills in the form and then they receive an Power Automate email, with details they filled in for confirmation, from an internal email account we select, which works great.
Step 2 (approval): Then an approval process start and goes to their line manager for them to approve or reject. However, this is where I wish to add the same email address from step 1 and by design, I don't have option to add preferred email.
Note: I know send email with options exist. However, with that approach, you would need to grant access to the account where you wish to send email from. The problem is you may not wish to grant Power Automate access, especially if it is a shared inbox!
The best solution would be to have the option "From (Send as)" so that you don't neccassairly grant access apart from sign in that account to have the sending priviligies.