In Mythic plus the progress bar for magma wave ends roughly 5 seconds too early. If you have countdown checked it counts down to 0 too early. Then messages, specifically the hide warning happens as magma wave is going off so you have no time to hide should you listen to the message warning.

I did not time it exactly but as far as I can tell it is off ending 5 seconds too early. It may be more but I do not think less.