Tried select2-4.0.5 with jquery-3.4.0 and started to encountered strange bug with focus logic. Especially multiselect widget was behaving very oddly (input focus disappeared, keyboard up/down started to change selected value in the field (no popup selector shown)).

Reverted back to jquery-3.3.1 and strange behaviours seemed to fix themselves. So I concluded that there might be some compatibility problem going on.