We often use custom templates for the data grid columns, the problem is that by default these columns don't have a tooltip from column value. As I see in the implementation this is because of this line in the datagrid column rendering logic:
var title = column.Template == null && this.ShowCellDataAsTooltip ? $"{column.GetValue(Item)}" : "";
Would you consider modifying the condition, so it still shows the tooltip even if Template is not null. Something like this:
var title = this.ShowCellDataAsTooltip ? $"{column.GetValue(Item)}" : "";
In normal situation when a column cannot get a value, title will be empty string, so everything works as intended, but when a column is linked to a property and has a Template it will have a tooltip.