Normally, 1 Baht coins are made from non magnetic copper nickel.
Pure nickel and some versions of stainless steel are magnetic.
I am guessing
that the slightly brown coin may be made of very low nickel (only) stainless steel. The slight admixture (guess 3-5 % ?) of nickel in an iron coin would whiten the alloy, and make it far more resistant to corrosion. Such an alloy would be magnetic.
I must admit, I have never encountered such an alloy for coins, so perhaps it may be a test alloy coin. Perhaps it would be advantageous to put a few hundred thousand coins into circulation, and test them for wear and corrosion in circulation. They could be easily found by magnetic methods for extraction from circulation for subsequent examination and testing.
You may have a scarce to rare coin.
See if you can get your hands on as many other One Baht coins from dealers' junk boxes, and do a magnetic statistical study on the alloys.
An easy and interesting piece of numismatic research.
I note the green tinge patch on the darker coin.
Perhaps other members here in the CCF may join into such a research. I will do my bit.