I am pretty sure that I saw someone else say something about this, but for the life of me I can’t find the post. It is an interesting theory so I want to hear ideas.
When BAM was imprisoned, it affected the entire land and created slave form among the singers. It created a new state of being for them.
What if it also affected the spren of the Knights Radiant?. What if before the imprisonment of BAM, the bond could be broken without killing the spren, only some pain on their part. BAM getting imprisoned changed the land and the Knights didn’t know anything had changed. They had the plan to end their oaths but they didn’t realize it would kill the spren.