TL;DR at the end.
At first I thought since it was this patch, it couldn't be the portrait causing minimap targeting on self-cast, because Riot said they fixed a thing where all abilities were doing it, but apparently globals and semi globals still do it. I tested a lot of things out for nearly 3 hours last night, and nothing else worked. Since Nami's ult is really long range, it still gets affected by minimap targeting, unlike normal skillshots like her bubble did up until this patch.
You were actually REALLY unlucky, because if you had just casted the spell instead of releasing the cast indicator, your ult would have gone in the proper direction, because it was over the XP bar, which doesn't self-cast. But you moved your mouse very slightly when you did cast it, and the casting bar came up slightly after. When you moved your mouse slightly while canceling the indicator, you were skating along the outer edge of the portrait glitch area as well, so you BARELY touched it, and you weren't there very long at all, but it does trigger like that.
I tried tweaking some settings to see if minimap casting can be disabled to try to fix it for you, but only minimap MOVEMENT can be disabled. ._.
TL;DR
It's just the portrait glitch causing minimap targeting like the first guy said, though it doesn't target relative to where your mouse is on the portrait, it actually fires relative to where you were coming from, so it would fire it where you were moving from if you're in movement command, and where you're facing if you're not in one.
