PVZ Battle For Neighborville - Online Co-op & Split-screen Issues

Split-screen functionality doesn't work when trying to join a friend's Giddy Park session who is already in split screen themselves. So 2 people on one console trying to join 2 people on another.

To explain better, My account (primary) has PSN (paid) and I have a secondary account (PSN account, but not a paid account) on my PS5 that my wife uses when we play together.

The two of us can play split-screen online together. The two of us can also be invited to Giddy Park by a friend, join his session, and play online with him. 

However, the two of us cannot join my friends session if he is split-screening with his wife. If he is playing split screen, the game takes away the option to send out invites for people to join.

This exact scenario plays out the same way if I am the host. Our friend and his wife can join my session while the two of them are in split-screen, but it then won't allow me to start split-screen with my wife.

This doesn't appear to be a PS Plus issue since the secondary (non paid online) accounts can play online without issue in any other scenario. It seems to be specifically a split-screen issue.

Why on earth is split screen disabled in this scenario?

*FYI I posted this in the tech issues thread previously, but never got an answer. So trying in this thread as well.