External VTT subtitles break playback in MS Edge

[BucherTomas]

Reproduce by trying to play Tears of Steel (External Text) test vector from Shaka's Azure Media Services asset group in Edge browser. It will download the manifest, a couple of segments, but then the player stalls and no video will start playing.

We ran into the same issue with our own implementation of subtitles loaded externally. Embedded subtitles in mpd manifest with their own Adaptation Set work fine in Edge, however. External subtitles work fine anywhere else but Edge, even in IE 11.

The only difference I can see here is that verbose logging writes to console:
(text:1000000000) all MediaStates need Period 0, but not all MediaStates are idle
from streaming_engine.js and no fetchAndAppend events are carried out for the text track. This line is missing in the log for all browsers where it works properly.