Your tooltip works because when you hover over a stack of overlapping marks, you're just showing the tooltip for one of them that happens to be on top.
You have multiple Client IDs per row, e.g. 2 different ones for YouThrive, so you need to use a table calculation, e.g. [Average LOS]=WINDOW_AVG(AVG([LOS])), add that to Measure Values, and set it to compute using Client ID. See image below and TWBX attached.
Zach...thank you! This was perfect!