Is this expected behaviour?

Using XPP 9.4 in CSS mode if I have:

para::before { counter-reset: tester 5; content: counter(tester); }

then I get the 5 output before the start of the text, however if I have:

para::after { counter-reset: tester 5; content: counter(tester); }

then the output is zero.

I don't understand why I can't load a value into a counter in the "after" selector - am I missing something?