"Jump to Specific Time" does nothing, while click on timebar works

If I stream from a digital PVR I have I can click on the timebar along the bottom and jump along the stream (in either direction).

If I use "Jump to Specific Time" (ctl-T) and set a destination time, nothing happens.

Doing a network trace shows that in the former (working) case an HTTP request to set a new Byte Range is sent, whereas on the latter case nothing is sent at all.