In Tools > Customize > Menus or Context Menus, the Modify split button has an 'Add Submenu' entry which should be removed from there and added as a separate button, similar to what we did in bug 82840.
With the group button now only have a 'Rename' button in it for these two tabs, a 'Rename' button should likely be shown instead of the 'Modify' group button.
Muhammet Kara committed a patch related to this issue.
It has been pushed to "master":
tdf#103160 Add 'Add Submenu' button in Customize dialog
It will be available in 5.3.0.
The patch should be included in the daily builds available at
http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More
information about daily builds can be found at:
Affected users are encouraged to test the fix and report feedback.