Steps to reproduce:
On OneDrive, multiple files can be uploaded at once. However, the prompt that appears does not let me select multiple files. I have tried compatibility mode to no avail. Safari allows for multiple file selection successfully.
When uploading files using the "Files" button, it should allow me to select multiple files. The macOS prompt is different between non-working and working, as seen in the images below.
Orion, OS version; hardware type:
Version 0.99.111-beta (WebKit 613.1.12) on macOS Catalina 10.15.7 Intel
<Copy/paste or drag and drop to upload images or videos (up to 20MB)>
The "Files" button has always allowed me to upload multiple files on Chromium and Safari.
Proper Safari prompt: