To be honest the first approach should be to contact IMSI support IMSI support page
and complain about the 'programming error' in designer, which prevents you from using layer sorting. The fact that one is given the option in designer to using sorting, suggests to me it should work, but is faulty.
As a workaround, it can be done via deluxe, but is a bit messy and not worth the effort if it is needed on a regular basis. Below is one way to achieve sorting (not saying its the only way).
1). Create a blank file from your normal template (metric or imperial), and set line styles and whatever else you normally set, BUT only have one layer (Layer 0), NO blocks, only standard one paper space, basically this is a blank canvas to use as a base for all files. Save as 'Base_file.tcw' (or whatever name).
2). Either - use 'Save as' to save as your project name, and add everything else, all layers, paper spaces, blocks etc.
OR - open an existing file with everything already in.
3). In Design Director, Sort the layers, click the first layer, then shift click the last layer to select them all, whilst still selected CTRL + click layer '0' to deselect just that layer, right click and choose Save to file, save the layers file to desired location. Save and Close the file.
4). Open the base_file saved earlier and use Save-As to save a new name like 'Project_name_2' (this simply preserves the base file for future use). in design director- layers, right click and choose Load from file. find the previously save layers, if one used the default name it will be something like LayersTemplate-(a number).lrs, and load the layers, they will (should) appear in the sorted form.
5). Go to File menu - Extract from, in the dialog, find your project file, and in the right hand window tick 'contents; then open. Save the file.
Opening it in designer should retain the sorted order. But like I said its a messy way to do it, and its no good if you are constantly adding / subtracting layers and resorting.
This next bit is just a guess based on my tiny bit of programming knowledge. I'm guessing what is basically happening is, TC normally retains internally, the order one originally creates the layers, the sort is just a flag which, on opening the file tells TC to sort the layer list, because the sorting is faulty in designer, TC is not reading the flag, so is not sorting the layers when opened in designer, the above procedure for loading the layers into a file, simply creates the layers internally in the correct order to start with.
Hopefully someone else will have a simpler solution.