Can you provide a link to the tek you are following, please.
You only freeze the mimosa powder if you have rehydrated it, probably in the acid phase of an acid/base extraction, and it will freeze as part of a blg block of ice, the idea being the hydrated cells will rupture, releasing more of the precious spice. To be honest, this seems an awful lot of effort, time and energy, for marginal, at best, gains. I have stopped doing it myself.
Sounds like you basically did a straight to base, with super cold powdered bark. You could have given your bark and lye longer, and with more(careful) mixing, before adding the solvent.
Did you take a picture of the crystals before they dissolved? How was the yield? Roughly how much solvent did you use? Am I right in thinking you only did this solvent 'pull' one time?