Thanks for reporting. The issue is caused by access restrictions introduced by the app isolation. It may be a wider problem than just importing contacts. An internal bug has been filed about this.
While one cannot import contacts from “miscellaneous folders” (e.g. /home/defaultuser/tmp), it still works from standard folders like /home/defaultuser/Downloads and /home/defaultuser/Documents.
VCF files sent to a phone via Bluetooth end up in folder /home/defaultuser/Documents.