iPad mini 7 split screen apps not meeting at center

Can you enlarge any of the apps to make them take up more space?

If you can't then that's the reason they won't meet in the middle, they don't have a supported sizing for the half screen space.

Try a different App like the Mail or Safari app instead of one of the apps. And do the same thing, it should let you extend the mail app to meet the other app at its edge, but not the middle since it appears the Monopoly or Sticker Hub or both don't support the half screen size.

For example: the Bluesky app below cannot be expanded to half screen, but the Safari app can be expanded past half screen and so engages the split screen bar at 2 thirds.

Some Apps are designed explicitly for iPhone - and the screen presentation may be designed for the size and aspect-ratio of an iPhone. Apps intended for iPhone often do not support re-sizing of an App window to utilise the much larger screen dimensions of iPad - and are therefore technically constrained by their design.

By contrast, some Apps are designed to be used on both iPad and iPhone - and will allow App windows to freely resized to make use of the available screen.

Apple do invite submission of constructive feedback, feature requests and comments via their product feedback pages:

Feedback - iPad - Apple

If you would like to see changes or modified functionality in a future release of iPadOS, this would be the most appropriate channel through which communicate your request.