In V11 this should not autoformat, since lazy is still a reserved keyword there, but will be only a plain function in V12.
The VSCode extension already ships with a V12 formatter for testing purposes and there seems to be something off with your configuration.
Do you have allowBuiltInFormatter set to true in your VSCode’s settings.json? Also do you have an invalid path set as platformPath? Try to unset them both
"rescript.settings.platformPath": null,
"rescript.settings.allowBuiltInFormatter": false,
If that does not work you may need to point the extension to the correct platform path, e.g.:
"rescript.settings.platformPath": "node_modules/rescript/darwinarm64",