This is a bit unusual and very confusing while I double check the modeling and results. Please try to fix it.
Which section database is this Pipe section taken from? Which STAAD.Pro version are you using?
Dutch database. Latest CONNECT edition 21.00.01.12
This issue has been reported for the development team as a Defect #799765 in order to be fixed in the upcoming STAAD.pro versions.
Answer Verified By: AbdelRahman Hamzah
The problem is still there even after the new update.