I have made some experimentation. It seems that exporting and importing works when using the XML format.
But there are issues when using either CSV or Excel format in exporting and then trying to import the same file. I am exporting few examples to get a good template for the file, so that I can use the same template and mapping when actually importing data from other sources.
It seems to work for test cases (text), but there are some issues when using test cases(steps).
Exporting to test case (steps) in Excel format seems to lose some data regarding the steps
Exporting to test case (steps) in CSV format seems to contain the test steps, but importing it back to TestRail does not seem to work. The test steps are not shown properly, but everything is in one test step.
Have I forgotten something, or are there some settings that I am not using correctly, or are there issues in the functionality.
You can easily try to reproduce this issue.