Why would Jill disappear? Even if she isn´t your love interest, she is still Bella´s best friend. Would feel kinda weird if she suddenly stopped appearing.
There are 2 outcomes with Jill when following the Ice Queen path: surprised or hurt. If you want the surprised face then all you have to do is not to kiss her during the tennis date (because you´re unable to evade the date) and simply not doing the romantic things with Jill. It´s not that hard to obtain.
I didn't kiss her, I was friendly with her but did not make any romantic approaches at all. This was confirmed when I was given options towards the end and I was just offered BELLA or THE OTHERS - not JILL.
But after the breakfast sex scene with BELLA, JILL still turns up with two coffees and looks upset when she spots BELLA and the main character together.
Then in the following free roam, BELLA texts and says
"MC. Did you hear anything from Jill, by any chance? I've tried to reach her for a few days, but she's not answering her calls or texts. I'm getting kind of worried."
MC then replies saying that he saw she was online on Rooster but she didn't write anything. He wonders whether there is something about the date with Tybalt.
BELLA then says "I don't know. I wrote and asked her about it, but I'm getting nothing. I haven't seen her around the library or school either so far this week"
So whatever way you play it with BELLA, it does seem to leave JILL upset and she goes off grid.
Even if you have not pursued a relationship at all with JILL the outcome is still the same, she gets upset and disappears. One possibility is that she is upset with what happened with Tybalt goes to visit BELLA for consolation, sees that BELLA is with the MC and feels that she has lost her best fried (BELLA) too.
Either way, I can't see that it is possible to complete the BELLA path without JILL becoming upset and disappearing.