Here's some useful steps that helped me get this feauture back:
- Open: opera://flags/#scrollable-tab-strip in a new tab;
- Go to where it says "Scrollable tab strip";
- On the drop down menu to the right of it, click "enabled";
- Hit the "reload button on the bottom right or if it doesn't show itself, RESTART your computer;
- Fire up OperaGX, try it out and see if it works.
Enjoy your feature again like before!