The submitted value [value] in the [name] element is not allowed.

Same problem here. Using std checkboxes it's working but not with Select2.

Think I found the problem.
We should be running an extra process facets_exposed_filter_remove_validation (facets/modules/facets_exposed_filters/src/Plugin/views/FacetsViewsPluginTrait.php) which is meant to unset($element["#needs_validation"]);

However for some reason when using select2 nothing gets created and thus this process does not get attached or run, meaning that validation is still enabled.

Bumping this to major as it affects this basic functionality from working as expected.

Core Views renders its "exposed form" before it fires the query.

Because how facets work, we only know the values after the query has been executed.
Thats why we re-render the exposed form again a second time.

That's why, if you are debugging, the first time you'll see an empty #options list, which is re-rendered the second time.

I dont have time to debug the select2 implementation, but this info could help somebody debugging it ;)

I have the same issue when using only facets + views + exposing sorting options in the view.
Not using select2, nor Facets exposed filters (Experimental).
The results are sorted as expected, just the warning is there.

My sort fields were called "name" and "changed".
Debugging this I found that the valid values were "title", "z_date" and "relevance", but not sure where this keys are coming from.
Might be a different issue than the one you are having.