Why did my caramel come out grainy?

Your thermometer should be fine, the graininess probably has more to do with technique than temperature. There's quite a bit of controversy as to when and how to stir and what to do about sugar crystals that form along the sides of the pan.

Just to be sure about your thermometer, test it with boiling water. If you're at or near sea-level the thermometer should read 100C or 212F. If your thermometer is fine, then we need to take a harder look at technique.

Edit: Looking at your recipe and considering your change in cooktop, I can only see a possible problem if the candy reaches 245F more quickly than you are used to. Try lowering the temp a bit at that stage so that it takes longer for the candy to reach 245F. It may take some getting used to, but induction should perfect for candy making.

It pretty much goes without saying, so I am assuming no considerable change in your ingredients.

I noticed that you're stirring constantly. Generally with candy you want to stir as little as possible, since it causes sugar crystals to form in the syrup. In addition to making the candy grainier, they reduce the overall lifespan- even after the candy has cooled the (larger) crystals will continue to grow, causing the candy to slowly revert back into flavoured sugar.

Once the sugar is fully dissolved, you want to disturb it as little as possible. You will still have to do some stirring, otherwise the milk solids will burn. It'll help a bit to mix in the corn syrup at the same time as the sugar. Corn syrup is an invert sugar, meaning it gets in the way of crystallization and slows it down. If your caramel is still coming out grainy, try adding half a teaspoon of lemon juice. Your candy will be slightly gummier, but the acid in the lemon will inhibit crystallization and reduce the graininess.

I own a bakery and made Caramel weekly. I finally purchased an induction burner for the back of the bakery so as to not have to carry boiling sugar down a long corridor. First my staff and then myself experienced repeated failures, i.e. graininess every time we used the induction burner. We finally went back to the gas range and everything went perfectly well. Not once was it possible to reproduce the smooth dark, rich caramel we used at the bakery when using the induction burner. I wish I knew why, I used the same pot, the same technique, same temperature, and ingredients. A mystery till this day but also a very dependable result- unusable grainy mess. I hope you find your answer. If you had previous success making caramel, and changed nothing other than the induction burner it may well be something with the way an induction burning heats the sugar crystals not anything you did incorrectly. I can only speak to my own experience but I will never attempt caramel with an induction burner again. I hope this helps.

I solved the problem of grainy caramel made with condensed milk by: taking the grainy caramel off the cooktop and decanting the entire batch into my high power blender (2200w) for 3 minutes on the highest setting. Not only did the graininess disappear completely but the heat generated by my amazing blender continued to cook my caramel. Now the caramel filling for my banoffee pie is PERFECT and ready for the bananas and whipped cream! I've never before made caramel with condensed milk. Next time I'm just going to use my blender from the beginning.