VTTCue Not Shown in LG 2019

Hi LG Team,

We have issue regarding subtitle not shown. So, since we use Shaka player, video subtitle is processed as TextTrack - Web APIs | MDN. But we found that in LG 2019 the subtitle is not shown, while in LG 2022 and 2017 it is shown.
And when we try to debug the activeCues the text is available and valid, but still the subtitle text can not be visible in the device. Attached is screenshots of debug console and livestreaming of the device, in which we try to update some appearance related value of VTTCue attributes:

cue.line = 1
cue.position = 50
cue.size = 100

So, is there any related issue in LG 2019 device? And could you help us how to show VTTCue in LG 2019 devices?

Currently, only an external subtitle in the form of .vtt (WebVTT) is officially supported.

<video src="" controls autoplay width="960" height="540">
    <track default src="subtitle.vtt">