So we are looking at several potential issues, as Microsoft has upgraded requirements for shared Mailboxes, so at the moment what you are trying to do, it doesn't have access to the shared mailbox, due to potentially several things.
1. Make sure the service account has both Full and Send As permissions
2. Make sure both flows use the same connection reference or it will try to use its own (aka the bot), which of course won't work
3. If you need too, and this is usually what I do, I have the From set to my service account, and I give Send As to the account you have listed above
Check that out and pick which one resolves our or you decide to use. I usually do #3 myself as its easy (IMHO)
Also, you may have notice thing specifically due to the enhanced security requirements from MS in the last little while
If these suggestions help resolve your issue, Please consider Marking the answer as such and also maybe a like.
Thank you!
Sincerely, Michael Gernaey