This is similar to the bug contained in GDevelop 5.1.154. It should have been improved in the latest GDevelop 5.1.155, but the improvement may have been incomplete.
Also, I've confirmed that the latest browser versions of GDevelop also have the problem. I have contacted GDevelop about this issue.
If you don't mind, press ctrl + shift + i in the preview window to open the developer tools and attach a screenshot of that console screen here. (What I need is the 3rd line)
If the third line of the console screen shows "Three.js r146" then the extension is included in the game.
