Defragging an SSD on a modern OS just runs a TRIM command. So probably when you wanted to shrink the windows partition, there was still a bunch of garbage data on the SSD that was "marked for deletion" but didn't fully go through the entire delete cycle of the SSD.

So "windows being funky" was just it making you do a "defragmentation" for the purpose of trimming to prepare to partition it. But I don't really see why they don't just do a TRIM inside the partition process, instead of making you do it manually through defrag