Mandos knew dealing with Fëanor would be more complicated than others when the elf was confronted by the elves who had died crossing the Helcaraxë.
To the victims at Alqualondë, Fëanor had offered nothing but sorrow, humility, and genuine apologies. When confronted by those who died on Helcaraxë, Fëanor listened to their stories, cried with them...and then said, "My brother has always been unreasonable."
Absolutely nothing anyone could say (or, more accurately, yell) could convince Fëanor he shared even the slightest blame when it came to people crossing the Helcaraxë.
Within a week, Fëanor had managed to find the areas of the hall that humans were kept in. Then dwarves. Even though that should have been physically impossible.
In another week, Mandos was preparing an argument for why Fëanor should be released back into Middle Earth. Because he knew that the truth, that dealing with Fëanor was giving Mandos high levels of anxiety, would be humiliating to admit.
