This seems to be specific to your model as it works fine on my side.
I do not know how you “succeeded” to get this problem on your side … but here is the solution :
• 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