Filtering (filterable: true, filterLenght: 1) inside PT_SINGLE_SELECT is broken starting of 2.8.8 for.

The issue seems related to JS / HTML rendering (see attachments). What I can see is that the filter html element is expected to be within the parameter div, but it is actually rendered next to it. Thus filtering is not set up and does not work. Moving the element within the div before rendering resolves the issue. 

So either adjust the JS to select the filter element or render it at the expected place.