Hello,
I found a couple of discussions (some 2 years ago) about that subject but can believe it's a by design behaviour in 2019 since it makes the approval process completely unusable for users.
Let supposed I create a simple Flow that just sent the approval and update 1 metadata columns.
1) user receives the approval
2) user opens the document in a web browser tab with the link provided in the approval email
3) user switch back to email without closing the web browser and approve the document
4) the approval is sent but Flow will fail to update the metadata.
Any viable solution?