The specifying professional is responsible for the end attachment for uplift and uplift strength is dependent on the joist seat thickness. Typically, during the design phase, the engineer doesn't have this thickness, so how can they perform this design?

It is correct that the project’s Engineering of Record is responsible for the connection of the joist or Joist Girder bearing seat to the structure including uplift loading. Although the EOR may not know the thickness of the bearing seat angle, they can control this by specifying the weld size for this connection or by simply specifying the thickness for the bearing seat. For example, if the EOR specifies a 3/16″ fillet weld for the joist bearing seat connection, the joist manufacturer must supply a bearing seat thickness of at least 3/16″ so the specified weld can be developed. In this way the EOR can know the thickness of the bearing seat. FYI – it recommended that the weld size for a K-Series joist not exceed 1/4″.