Both observations are the known obstacles: OWA's importer speaks CSV rather than vCard on most tenants, and desktop Outlook's wizard reads only the first entry of a multi contact vcf while reporting success, the trap covered at length in the iCloud thread linked alongside. Two working routes past them:

The drag route, fastest when desktop Outlook connects to your 365 account: open the People view, make certain the folder shown is the Contacts folder under the 365 account rather than any local one, then drag the vcf from Explorer and drop it onto the contact list itself. Dropped vCards process every entry, all 300 land in the account folder and sync server side to phone and webmail, the wizard bypassed entirely.

The translation route when only a browser is available: convert the vcf to CSV using Google Contacts as the free translator, import the vcf at contacts.google.com into a throwaway or spare account, immediately export as Outlook CSV, then feed that CSV to OWA's People import which maps the Outlook headers automatically. Two extra steps, zero installs, same 300 contacts server side.

Verify either way by the phone test, the contacts appearing in the phone's synced list minutes later proving the server side landing, and run a quick duplicate pass if your mailbox already held some of the colleague's contacts from mail history, sorted adjacent in a list view by name.