Hi,
I have reproduced your issue.
It's a bit special from other entities since there are two button:
- The "NewRecord" button(New {0}), which is mentioned by you, is used to create a new record of Connection record.

- The "newConnectionOnSubgrid" button, which is next to the "NewRecord" button, is used to create Connection for the sub-grid. In other words, it is possible to create a connection not only for the sub-grid of the entity Connection, but also for the sub-grid of other entities.

And the second one is which you need to customize.
For example, If I hide the first button and publish it, the + New Connection button will still be shown in the sub-grid of Connection.
If I hide the second one, it will no longer be shown.
