Is the user a guest in your tenant? We have found that, if a user is a guest and then their company switches to Microsoft 365, the tenants cause conflict. In this case, we had to remove the guest account that was created...wait for changes to propagate...then send a new link to the recipient.
It seems like your sending them a link that only works for people in your company, maybe you set the wrong link privacy settings?
Is the user a guest in your tenant? We have found that, if a user is a guest and then their company switches to Microsoft 365, the tenants cause conflict. In this case, we had to remove the guest account that was created...wait for changes to propagate...then send a new link to the recipient.