Yeah, that can be a problem with magnetic sensors. Commercial buildings with steel doors run into this issue all the time. Usually the magnet part ends up magnetizing some part of the metal and this confuses the sensor part. Remember those old school experiments where you could extend a magnet’s field with bits of metal? This process takes time, which is why there’s a delay to failure.
Anyway, there are a couple of things you can try.
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first, make sure the sensor does still work. Sounds like you did that.
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if this is a reed sensor with a magnet in only one piece, that’s the piece you have to worry about keeping away from the gate/door metal. If the post is wood, sometimes just installing with the sensor on the gate and the magnet on the post avoids the problem. But once the problem has happened, it takes time to go away.
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usually the answer is a thin piece of plastic (not wood) between the magnet piece and the metal of the door/gate. So you had the right idea, just not the ideal material.
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it takes time for the problem to occur and it takes time for it to go away. Unfortunately once the other metal has become magnetized you can’t just get a new sensor or sensor position or barrier and start over-- you have to wait for the charge to fade in the gate. So you may just have to remove the magnet for about two weeks, then start again. And be aware that it might take a week or two for the problem to come back, making testing tedious.
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there are door sensors made for steel doors that use stiffer reeds and stronger magnets but they cost a lot more and you may have to move the antenna to a third piece so gets complicated. The barrier method is better.
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often weatherproofing the gate with a plastic seal will help a lot. It depends whether you like those aesthetics. But there’s no question that flakes of rust on a wrought iron gate will make the problem worse.
Anyway, it sounds like you had the right idea, you may just need to give the gate metal time to demagnetize. If you don’t want to wait, lifting both sensor and magnet piece up off the gate surface about 4" with temporary blocks will likely work, but most people don’t like the aesthetics. Might be ok for a temporary solution. Then in a couple weeks, replace the blocks with a thin plastic sheet.
Here’s an article that mentions the issue.