Subject line of first unread post displays with incorrect colour

With stock phpBB 3.2.x (prosilver style), when viewing a topic the Subject line of the first unread post is shown in black text to clearly distinguish it as shown in the screen capture from this board (attached).

On one of my boards that I recently upgraded to phpBB 3.3.0, members are reporting an oddity with this behaviour. The Subject line of the first unread post isn't shown as black text. But the oddity is if they refresh the page, it does show as black text as expected.

I can reproduce the behaviour on The demo of our latest version, phpBB 3.3 and have tried to use my browser's inspector tool to see if I can figure it out but so far it has eluded me.