Currently, it appears that ProtonVPN does not support multiple ProtonVPN users of the same macOS machine, while using fast user switching.

After fast switching to a second user who also launches ProtonVPN, any attempt to establish their own VPN connection is unsuccessful. If the second user's instance ProtonVPN remains open, this also causes the original user to be unable to connect if their account is later returned to.

Logging out rather than fast switching does not appear to cause the same issue. Nor does manually closing ProtonVPN before making a fast switch to another user.

Other VPN providers enable multiple users to establish their own VPN connections in this way, handling the making and breaking of connections while fast switching between accounts. The inability to do so is a significant drawback for ProtonVPN users on macOS if fast user switching has to be used for other reasons.