Essence Reaver Effect not working properly

The Essence Flare proc from Essence Reaver is not triggering if you cast another ability immediately after ult before auto attacking. First noticed this on Naut when trying to ult+Q combo, then tested with Ashe and Lucian, maybe 2 out 10 tries with lucian triggered it but once i spaced him out of auto range to test with Ult+E+auto combo then it would not trigger the Flare. This was all found and performed in Practice Mode.