Others have already mentioned that you shouldn't be sharing a master password, and that's definitely true. I'll try to answer the question more holistically:
RE: Sharing.
Nobody should be sharing a password manager account. If this is happening it usually means somebody doesn't want to pay for the seats to have individual accounts. You give each user their own account, and if you need more than one admin, you have more than one admin, but you shouldn't be sharing an admin account.
This would also apply if you're not talking about the pw manager's master password but something more like an AWS root account. That root account needs to exist, but its credentials should be a secret in the password manager vault that is shared with anyone who needs it (based on roles/access rules/principle of least privelege), and you should create additional administrative roles within the system you're securing instead of leveraging those root credentials (as much as possible) operationally.
RE: Bad actors
SSO is the answer here. You use a password manager with SSO integration, and you cancel the person's account the second they depart. At previous orgs they literally cancel the account while the person is still in the building and being walked down the hall to be informed that they're being asked to leave. It's definitely on the cruel side but if you're concerned or wanting to emphasize the security of the data first and foremost, that's what you do.
Both parts of the answer above involve $$$. Most services require paying for a higher tier if you want SSO integration (both on the side of the provider like Google Workspaces and the tool like 1Password), and both require paying for head count.
But this is the answer to "best practices". If there's a financial impediment that prevents you from doing these two things, then you can't use the "best practices". That's the unfortunate nature of the beast, and why there are so many online disagreements about the concept of an "SSO Tax".