Sorry for the double-post, looks like the "edit" period for my previous reply has expired.
I'm back with another update, on a different computer but with the same Opera GX version and account.
So, as expected, the workspaces I added a few hours ago on the other computer is automatically here (so >5 workspaces). However, trying to add more on this computer reverts back to the behaviour of replacing/renaming existing workspaces.
So I went to check the Music/Player functionality deep in general settings again, and it's already enabled (it looks like settings sync up very well on Opera 😁). So I toggled it to "Disabled" this time, and tried to add more workspaces again immediately (without any browser restarts etc).
And bam, it's behaving as intended again.
But I've grown to like the Music/Player sidebar thingy over the past few hours, so I turned it back on, and adding workspaces still behaves correctly.
So now it's confirmed that toggling the Music/Player functionality is one reproducible way to fix this bug regardless of whether it starts or ends on "On" or "Off", and does not require any restarts.
Hopefully devs can poke into this to find out the actual interaction to be addressed.
Open tab should be highlighted with much visible silver tone colour, keep closing wrong tab, very annoying.
Also "x" to close tab should have been on the right, why ? When the tabs open towards right, why on earth you'd put the close "x" on the left side ?
I love opera and the dark theme but these two things needs must be corrected.