If I have in the filter a SKU field named size, when the value is "S", it does not work properly

In the filter list, we have size selection:


, I have a value S1, which works OK but is strange. If this value is “S” and I select it, I get a display of all products and the Filters are not displayed. I need to click on SHOP again to get all the filters back and then I can select other values. If I select M and S together, it works. I get the selected products and the filter column is displayed.

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