We get that too; it's puzzling and confidence-sapping though it doesn't seem to cause any problem apart from the message and delay each time we hit play in Unity.
We're currently using Wwise 2019.1.2 with Unity integration - but the late November 2019 Unity update for Wwise 2019.1.5 (Wwise Unity Integration 2019.1.5.7093.1568) lists the following under 'resolved issues':
WG-45806 Optimized the processing behind the "Fixing clip durations of AkEventPlayables" pop-up, at
I suggest you try updating Wwise, Unity integration (and Unity itself if necessary, sheesh, all the layers before our own!) and post back if it fixes the problem for you.
Meanwhile I'll add a task to update our components to our next maintenance schedule, and hope it works for us too. ;-)