So this actually just means that any one of your blueprints detected it was going to perform over 1 million instructions. Note that it is instructions not loops that are counted. So every single operation you perform inside of a loop will reduce the number of iterations that can run without hitting that limit.
As I recall, a simple forloop that just executes does around 4 instructions, so right off the bat you are limited to 250k loops. Some nodes require several instructions. For example doing stuff with arrays like breaking and lerping all add instructions. It is possible 4.3 made one of your nodes cost 1 more or some other almost trivial under the hood change that could have pushed you over the limit.
How many things are there of “pieces” driving the foreachloop? Seems like it would still have to be quite a few to trigger this. If you can build with visual studio, all you have to do is increase the 1000000 limit found in scriptcore.cpp and recompile.