I have 2 SPO libraries with identical issue. Both libraries have been created a couple of months ago.
Issue is that when I click on "+new" & choose to create new excel file it starts to open Excel Online.
When I click on "+new" & choose Word file, desktop Word starts to open.
I would need for Excel files to be opened in Excel desktop
I've tested it in 3 browsers - FireFox, Chrome, IE - all have the same issue.
I've already activated options "open in application" under Advanced Settings & "open documents..." under "Site Collection features" is activated.
In top ribbon you can see option open in desktop app please click on it it will open in desktop application please find below screenshot