The screws are standard machine thread, not self tapping. The consensus here, is that the cause of the problem is dissimilar metals, which has induced corrosion between the screws and the block.
I have seen a hole tapped for course threads and then fine thread bolts forced in with an impact. And vice versa. Basically tapping as you go with force.
You might have to drill.
More than likely it doesn’t have Loctite on it when installed. Loctite would of prevented any corrosion enabling you to break it loose.
Loctite only increases the area of contact between the threads which is around 25% contact. Loctite increase that by filling in the gaps.
Drill it out and retap. Make sure the tap thread pattern match the new bolts you will use to reinstall.
I also remove stripped or bolts with broken heads all the time with left handed drill bits drilling counter clockwise right into the body of the bolt.