If you have been using LR 6 for a period of time already then something happened to your original converted LR 5 to LR 6 catalog that you had been using.
When LR 6 (CC 2015) converts an older LR catalog to work with V6 it never deletes any data in it and especially not keywords or collections.
Converting a catalog from a earlier version of LR to one that is compatible with LR 6 is always necessary and never requires reimporting image files. The process is as follows: Once you have consented to the conversion, LR copies the old catalog - copies and does not delete it and it can still be opened in the older version of LR by which it was created - and then creates a new Lightroom 6 catalog which contains all the images from the old catalog as well as any metadata (edits, keywords, collections, etc) recorded in it.
However, by creating a new blank LR 6 catalog and reimporting your images you are starting from scratch, without your old metadata.
So if I go ahead and consent to the upgrade again using an older LR5 Catalog out of Time Machine should it just upgrade that Catalog as you have described?