There is a bug in eZSelection field rendering.
Inside the file "content_fields.html.twig" line 166 should be:
"{{ fieldSettings.options|slice(selectedIndex, 1) }}"
instead of:
"{{ fieldSettings.options[selectedIndex] }}".
The bug is easly exploitable creating any content with at least a field of eZSelection datatype (multiselect) and trying to render content, you will se wrong items.