Rendering variants of U+3127 Bopomofo Letter I

[Dreiheller, Albrecht via Unicode]

Hello Chinese experts,

The Letter I in the Bopomofo alphabet (U+3127)  has a two rendering variants, a vertical bar and a horizontal bar.

Can anyone please tell me the context criteria, when should which variant be used?
Is it VR China using the vertical form (like in font SimSun)  and Taiwan using the horizontal form (like in font  PMingLiU) ?

Thanks
Albrecht
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