Bravissimo fitting for the second time - have I got the right size ?

I made a post earlier about when I tried on the 34F bras at bravissimo, I did feel that the underbust part is a bit tight. they said it was probably because i was used to wearing a band size too big for me (38DD) and i need to get used to the right size. So I ordered a 34F bra and it was too tight underneath my boobs.

I went back to the shop and exchanged the bra to 36F. The fitter this time originally suggested me to try on 34FF, and it still felt too tight underneath my boobs. So I asked them for a 36E (when I got the fitting done last time they told me my size was 34F/ 36E). But they suggested me to try on 36F and it felt more comfortable so I got that one.

My question is, after wearing it today, i noticed that the back of the bra would still raise up a bit (my old 38DD bras did raise up a lot at the back too) - does that mean the 36 band size is too loose for me?

There is also a small gap in the cup, where the strap and the cup connects, near my armpit.

FYI the calculator on this sub suggested 36FF/34G to me. when i tape measured my loose/snug/tight underbust are: 36.5/35.8/34.5.