I did forget that AjaxSelectFieldWidget does not do batching. So once you do a search the result is probably ok, but really you want batching/paging especially if your query string is empty. I actually brought this up awhile back Batching AjaxSelectFieldWidget - #2 by MrTango so batching is supported in select2 but not with the mockup implementation. It really is too bad this doesn't support batching because even if you get the Autocomplete widget to work you are lacking design consistency on forms that mix this and the Ajax select2 implementation.
Side note: https://dist.plone.org/release/5.2.1/versions.cfg pins a version of plone.formwidget.autocomplete that is incompatible with Plone 5. I don't know what repo governs this, or where to report it.