We cannot open multiple items in multiple new browser tabs in one environment.
When we try to open multiple selected items at once, only one of the items is opened in a new browser tab.
In another environment, this does work.
We have tried different browsers (Chrome and Edge) and the behavior is the same.
This is due to the browser's popup blocker. It allows to open one additional tab but not multiple at the same time.
The popup blocker can be set for each URL separately.
To resolve the issue you have to allow popups.
This can be done in Chrome as follows: