thanks for your answers, I didn't realize there would be several ways to write the syntax for this but I was also able to get it working by using this 0%/100% notation (whereas using the from/to notation did not work):

@keyframes rotate {
  0% {
    --angle: var(--offset-angle)
  }
  
  100% {
    --angle: calc(var(--offset-angle) + 360deg);
  }
}