You may have noticed that you are unable to split your tests by timing for builds that are triggered by Git tag. This is expected behavior and is due to how the execution occurs.
How Does This Work?
When splitting tests by timing data, the "Downloading Previous Test Results" step is required. Since this step is only present on Git branch-triggered builds, there will be no timing data present to accurately split the tests.
What Should I Do?
One option would be to change how you are triggering your builds, but If triggering builds by tag is required for your use case, you will need to change the --split-by= method you are using.