Implying she's bi is the localization's way of writing around "she's heavily implied to be into girls, but only has S support endings with guys".
Because, again, in the Japanese version she's not actually gay. She's just got a temporary thing with girls, but she'll end up with a guy, obviously.
Anyway, to understand why this is the case, we need to take a look at Soleil's personal skill, Sisterhood.
What this skill name is evoking is the "Class S" dynamic. S here standing for sister. To explain it overly simplistically, class S was a socially acceptable way for Japanese women to have romantic relations.
At its best, it'd let women be gay in peace, not really considered "properly" married, but at least not actively criminalized like in the West.
At its worst, it was seen by most people as just "fake" relationships, "practice" for the "real thing" (a relationship with a guy). If you've seen yuri manga deal with this stereotype/prejudice, this is why. It's a thing.
And, of course, when Fates talks about Soleil's interest in women, her sisterhood with them? This stereotype is the one they mean. The clearest evidence that this is how the game treats Soleil's attractions - as just a confused teenager playing pretend - is her Support conversations with her mother.
Her mother, of course, can be one of a few people, depending on who Laslow married. But here's her Support with Corrin as her mother.
See? She's even extolling the feminine beauty of her own mother, in exactly the same way as she'd flirt with any girl. And of course, Corrin (and all her possible mothers) simply tell Soleil to stop teasing them.
Because Soleil isn't gay, she's just confused. This is the explicit intent of the script and how she's written.
Which, by the way, with the English Translation choosing to say "oh no she's definitely bisexual", they created the unintended subtext of... hey, uh, does Soleil have an incestuous crush on her own mother?
Because this support only reads as a genuine crush, as genuine flirting, in English. In Japanese it's just a confused teenager.