For one of our customers, the "Merge" button doesn't appear when selecting 2 Leads (not on the Home screen Ribbon nor the Subgrid Ribbon), while these buttons appear just fine for Account and Contact.

I have checked the following:
- Users have the correct access described here. Not that it matters a lot, as me (as a System Administrator) also can't see the button while testing.
- I've enabled the Hybrid Experience.

- There are no customizations done to the Merge button using the Ribbon Workbench. No enable rules ore display rules are present.
We are running Version 1710 (9.2.21063.131) online.
Any help would be highly appreciated.
Kind regards
Sven