Thank you for your model sent by email (on my side, I do not get this issue when creating such a railing).
Please use this workaround to get the expected result for the railing in your model :
• Double-click on the Railing cuboid to get the properties dialog box get displayed
• Go to Handrail > End of handrail
• As “Connection type”, choose “Nothing” and then choose “Knee”
• Close the Railing properties dialog box