Zend certified PHP/Magento developer

Select multiple options in a single attribute in catalog

Not sure if I’m doing something wrong or if it’s by design, or if it’s a theme. I’ve added a custom option (size) and enabled it for catalog pages, it does display as a filter on the left. However I cannot select multiple choices, for example choose to filter both “XL” and “S” sizes at once. When I click on the option, it immediately transfers me to that single size filter. Plus the picker looks weird: it displays both the radio button and the checkbox. What to do? Where to look?
Magento version 2.4.4
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