USF wrong timestamps

I'm using the example in Wikipedia. The initial timestamp is:

<subtitle start="00:00:06.000" stop="00:00:10.000">

After muxing with MKVToolNix GUI and extracting with gMKVExtractGUI, the timestamp morphs to:

<subtitle start="1666:40:00.000" stop="2777:46:40.000">

Repeating the process (muxing >> extracting) results to:

<subtitle start="1335604:27:24.396" stop="517975:34:09.423">