This has happened to me before - I ended up spraying a load of Muc Off silicone shine inside. I guess it made it super slippy inside the shifter and it got it going again. Worth a go if you’ve got it knocking around
If you haven't done it yet, I'd try a thin lubricant like Tri-Flow instead of the silicone spray. I'd be concerned that the silicone spray would leave a coating that would gunk up over time.
This has happened to me before - I ended up spraying a load of Muc Off silicone shine inside. I guess it made it super slippy inside the shifter and it got it going again. Worth a go if you’ve got it knocking around