This morning I tried using the app to add money to my Monzo account from my revolut account. The Monzo app switched me to the revolut app to approve the transfer. The money left revolut but was immediately returned to my account with a message that the transfer had failed at the recipient.
The message in Monzo was that revolut had rejected the transfer. Seems Monzo blamed Revolut and Revolut blamed Monzo. Another attempt also failed.
I then tried using the Revolut app to send money to Monzo and it worked immediately.
**** Update. Worked later that day so possibly a glitch in bank network.