Hi @Anonymous@Rebetcha,
Actually, we have already annouced the availability to use choice, lookup & people columns in SharePoint lists that have multi-select enabled on them. But notice that:
These changes are rolling out incrementally for SharePoint First Release tenants and are not generally available from SharePoint yet. So, if your tenant is already on first release, you should already have them and if your tenant is not on first release, to enroll please follow the instructions here.
Announcing availability of custom forms, multi-value choice and read-only attachments support for SharePoint with PowerApps
https://powerapps.microsoft.com/en-us/blog/custom-form-launch/
Check and change your tenant to first lease in Office365 admin center > Settings > Organization Profile > Release Preferences. I just tested myself. Before, I was unable to use the multiple selection column with ComboBox under stardar release. After I change it to first release, and wait for 24 hour. I create the app using sharepoint list connection, all my multiple selection columns are shown with combo box:


Regards,
Mona