When viewing retainer inventory in expanded mode, the numbers at the bottom right are underneath the last two slots of the page. The numbers are visible when there's nothing in those spaces, but putting items there covers them up.

Date & Time: At least since 4.25 (probably since 4.2 when they changed the shape)
General Rest of Stuff: Doesn't Matter
Platform: Windows 10 PC
Various resolutions, UI scaling, windowed/full screen/borderless windowed mode

Steps:
1. Set retainer inventory interface to expanded in character configuration
2. Open retainer inventory
3. Put items in the last two slots of any page of retainer's expanded inventory
4. Cry because those items cover up the numbers that tell you how full your retainer's inventor is