Patch 1.4.1, which was released earlier today, was meant to fix an issue surrounding players not receiving their Resplendent Sparks after killing Echo of Lilith in Season 4.
After downloading the patch, players who have already killed Echo of Lilith will receive the reward popup on whatever Seasonal mode character, e.g., Hardcore or Softcore, they first log in on — and the Spark will stay associated with that mode. This means that it's possible to lock yourself out of a Spark on your main in Softcore if you log in with an alt on Hardcore mode first.
Players looking to work around this problem should log in on the character they want to use their Spark on first — this will prevent any mistakes or materials from being stuck without a way to transfer them.