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
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