I also have the same problem.
Today when I open my outlook it switch from legacy to new interface.
After I selected the legacy option under help menu, outlook reopened but still under new interface. It doesn't switch back to the legacy model.
The reason why I can't switch to the new interface is because many of my old emails can't be accessed under the new version. my folders and everything are not the same as under legacy interface.
Please help!!! I can't do my work!
I m on
Licence: Microsoft 365 Subscription
Version 16.102 (25083120)