Seems since the new Call Appearance per line has been implented Vertical First does not work as intended.
I noticed a prior case that was resolved but for private lines, however for Shared lines this issue was not fixed.
2nd+ calls indeed still ring on the same button with Vertical First set, regardless of other SCA settings.
Only workaround I found was to revert to an older firmware prior to this new feature's release. (firmware 7.4.7).
Pulled my hair out trying to sort this one. If another workaround exists with the newer firmware's please advise.