"Use (create mode)" button is missing
Yes it's intentional that there are no links if there's mixed record types in a profile.

It's been like that for a LONG time:

https://issues.civicrm.org/jira/browse/CRM-1974

Also for reference: 
https://github.com/civicrm/civicrm-core/blob/5.15.1/CRM/UF/Page/Group.php#L353

UPDATE: Based on the comments, I think this is what is desired:


Create a profile that has just the membership fields.
On the manage contribution pages, on the profile tab for your membership contribution page, include the profile either at top or bottom.
When the person goes to buy their membership using your contribution page they'll see the custom fields from your profile.