TTML: wrong subtitle alignment

When I play a video with TTML subtitles (see example file attached), the subtitles have the wrong alignment. They should have horizontally centered, vertically bottom alignment (with the centered 80%x80% region). Instead they have top left alignment (within their region).

I suspect line 393: p_region->updt.align = SUBPICTURE_ALIGN_TOP|SUBPICTURE_ALIGN_LEFT; I'm thinking of the case that tts:origin may be parsed after tts:displayAlign and hence overwrites the previous alignment value, but cannot overlook all implications here.