PIP(Picture in Picture) is a Multitasking feature supported by iOS. You can change the subtitle style of the PIP mode in the iOS Settings app.
1
Go to Settings app > General > Accessibility.
2
Tap Subtitles & Captioning of MEDIA.
3
Tap Style.
4
<
Tap Create New Style….
5
After you set up a new style, tap Save.
Default
Apply the Style
BACKGROUND > Opacity > 0%,
ADVANCED > Text Edge Style > Drop Shadow