Checkboxes/radio buttons/selection options not showing a “tick” when selected

Hi everyone.
Desperate user here.
Using a new WP site with advanced custom fields for my posts.
When I insert a custom field so I can select between several options for the value upon creation/editing of my post, I can see the little rectangle and click on it but I am not seeing a checkmark. Same issue with radio buttons. Apparently, though, the selected values are being registered, just that I am not seeing the checkmark or the black circle on the boxes/radio buttons!
Anyone could please point me to a possible issue?

Thank you in advance.
Regards,

Héctor