In the code you posted, you have

<c-tooltip enabled="true" filter={myCustomTooltipFunction}></c-tooltip>

but the documentation says that filter is used to select only some of items of the tooltip, while the function that should be used to change the label is labelCallback, so it should be:

<c-tooltip enabled="true" labelCallback={myCustomTooltipFunction}></c-tooltip>

The documentation also includes an example of that function being used, see this link.