I think my chocolate fudge is seizing?

new here and amateur home cook.

I'm trying new fudge recipes and got one down that works extremely well when I'm not using chocolate. When I use chocolate, it... Seizes I think. It becomes crumbly without becoming crystalized.

Recipe is:

1. Bring 6oz. evaporated milk and 1.5c sugar to a boil until 224°F (I live a mile above sea level)
2. Pour it onto a greased pan (greased with vegetable or canola oil) that has been sitting in the freezer and throw that into the freezer asap
3. In a stand mixer bowl, add 5-7oz. of morsels (semisweet chocolate, butterscotch, peanut butter, etc), 6 tbs. of room temp unsalted butter, pinch of salt, and dash of vanilla extract. Set aside.
4. When the syrup is at 120-130°F, pour into the bowl with stuff in it and mix on low until combined. With the peanut butter morsels, I was able to let it go for a while and made some night light and fluffy fudge.

The chocolate fudge, however, comes out crumbly for some reason. I use basic semisweet morsels. One instance was clearly the chocolate seizing; when I pressed it into a pan, all the fat from the butter seeped to the surface like a sponge. Another time, I put the butter in the pan with the milk and sugar, and it still became crumbly through the fat didn't seep out almost at all.

Help? I've tried a lot of testing on the sugar syrup up to this point practicing the recipe and have since had the chocolate fudge attempts fail everytime.

EDIT: Solved! I simply pre-melt the chocolate morsels, make sure the syrup reaches 50°C/120°F, mix together and incorporate butter after the syrup and chocolate are mixed fully! It's been consistently working well.