Time format legend

HI bdel ,

According to your describe, I try to reproduce your problem.
I created a time column and set the format to decimal. Here's a screenshot of my test:

1.Create a table visualization , use duration column and value column.

2.Create a new calculate column using DAX

Column = TIMEVALUE(FORMAT('Table'[Duration],"hh:mm:ss"))

3.change type of column format

4.Add the column into table visualization

5.Final output

From my test, your DAX formula is right. If you still can't do it, please provide your documents and hide your personal information.
In order for you to solve the problem faster, you can refer to the following documentation

How to Get Your Question Answered Quickly - Microsoft Fabric Community

Best Regards,
Albert He