Since the magnet is said to be attracted to the weld alone, means the filler metal is in question not the material.
You can do a PMI test. I'm sure you can find someone from an NDT contractor to do this which should not cost too much.
It's a simple portable equipment that can will help you assess if the weld metal is a carbon steel or stainless, as PMI provides indication of alloy content of certain materials. It
![cry :cry:](/f2/epx.png)
can even tell you if the material under test is SS TP308 or 316, etc. it is easy to use so you can rent the equipment for a day and do the testing yourself.
for example:
https://www.ashtead-technology.com/page/positive-material-identification-pmi-equipment-rental-hire