Having been involved with pressure vessel fabrication, the problem is the cold work introduced during fabrication, and ferrite level in the weld metal(s). You are correct that the material should have been solution annealed after forming, welding is acceptable without solution anneal.
See the excerpt below from the Carpenter web site;
Austenitic (nonmagnetic) Stainless Steels
All austenitic stainless steels are paramagnetic (nonmagnetic) in the fully austenitic condition as occurs in well-annealed alloys. The DC magnetic permeabilities range from 1.003 to 1.005 when measured at magnetizing forces of 200 oersteds (16k A/m). The permeability increases with cold work due to deformation-induced martensite, a ferromagnetic phase. For certain grades such as Types 302 and 304, the increase in magnetic permeability can be appreciable, resulting in these grades being weakly ferromagnetic in the heavily cold-worked condition. The susceptibility of a particular grade to becoming ferromagnetic when heavily cold worked depends on the stability of the austenite, which, in turn, depends on chemical composition and homogeneity. This is described in the article "Stability of Austenite in Stainless Steels" by C. B. Post and W. S. Eberly, published in "Transactions of the American Society for Metals," volume 39, (1947), pages 868 to 890.
The effect of cold work on magnetic permeability is illustrated for several austenitic stainless steels in Figure 1. The relationship between ultimate tensile strength and magnetic permeability is shown in Figure 2. The rise in permeability correlates well with the increase in tensile strength or work-hardening behavior, which is another measure of austenite stability. The differing performance between grades is a reflection of their composition. In particular, nickel increases austenite stability, thereby decreasing the work-hardening rate and the rate of increase of magnetic permeability. Consequently, the higher nickel grades, such as Carpenter Stainless No. 10 (Type 384), exhibit lower magnetic permeabilities than the lower nickel grades such as Project 70+® Type 304/304L when cold worked in equivalent amounts. The high-manganese, high-nitrogen alloys, such as Carpenter 18Cr-2Ni-12Mn, are also noted for maintaining low permeability after heavy deformation.
The magnetic permeabilities achievable in austenitic stainless steels are very low compared with conventional magnetic materials such as silicon-iron alloys. Therefore, their non-magnetic behavior is more of a concern. Certain uses such as housings and components for magnetic detection equipment used for security, measuring and control purposes require that the steel be nonmagnetic. That is because the presence of even weakly ferromagnetic parts can adversely affect performance. Unless the austenitic stainless steel parts are used in the annealed condition and are not subjected to deformation during use, a higher nickel grade would be a prudent choice assuming it offered the appropriate corrosion resistance and strength.
For a given grade, the magnetic permeability can vary significantly depending on the chemistry and degree of cold work of the steel. Often a particular lot of an "unstable" grade such as Type 304 can perform satisfactorily. If the magnetic permeability of an austenitic stainless steel is of particular concern, it can be measured by relatively simple means as described in ASTM Standard Method A342.
