Same thing here. Previously I had an S7 that never had the option to share my musics on insta stories. Now I have a Note9 that still not showing the option to share my musics on stories, both of apps are full updated and I'm even a beta tester of instagram. The funny part is that in a old iPhone5s the share option appear normally.
I've already asked for some friends to log in my account on their devices to test if this feature would work, and for my surprised it worked normally on a S8 that doenst even had the latest version of spotify and instagram installed.
Hope you guys can help me, because has been almost an year without anyone knowing how to solve this issue.
Already cleaned my cache, unnistalled both apps, more than once, and all this kind of things and none of them worked at all.