I think I found a way:
override the display: [ none | block ] with opacity: [ 0 | 1 ] + height: [ 0 | auto ]
.acf-field {
transition: all .5s ease-in 0s;
}
#acf-form .acf-field.acf-hidden {
display: block !important;
opacity: 0;
height: 0;
}
#acf-form .acf-field:not(.acf-hidden) {
opacity: 1;
height: auto;
}
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Inspired by this and this (as partial answers)
Note: I just figured this out, I’m not yet sure if there are any downsides to it.