Every single time I reload the page the other characters are a broken rectangle, so experimentally it's 100% of the time I'll need that character. What I want is to not see a broken rectangle. One can't say that U+76F4 is useless when it clearly is serving a use for my computer.

On default modern Android I get the same: Only U+76F4 renders, not the other two.

OP means it's the only one that renders at all. It could be wrong but the other two are just missing-glyph boxes which are definitely wrong. Those code points appear unusable as a practical matter.