I am not seeing that behavior. You might want to check your Lightroom preferences, on the General tab, default catalog, and make sure it is set to open the most recent catalog.
Not seeing this. My LR 5 catalog was is on my D: drive, a separate/physical SSD, in a folder called Lightroom Catalog with a subfolder called Lightroom 5 Catalog and the actual catalog name as Lightroom 5 Catalog.lrcat.
When opening LR 6 for the first time my LR 5 cat was converted to a LR 6 cat and placed in that same Lightroom 5 Catalog subfolder and naming it Lightroom Catalog.lrcat, No version number. I then wanted to keep the 5 and 6 catalogs separated so I created another folder on my D: drive called Lightroom 6 Catalog and copied the converted LR 6 catalog to that other folder and renamed it Lightroom 6 Catalog.lrcat.
I then opened LR 6 and went to preferences and chose Other in the drop down for "When starting up use this catalog" and pointed it to the D:\Lightroom 6 Catalog subfolder and the D:\Lighroom 6 Catalog\Lightroom 6 Catalog.lrcat file. Then I selected Restart Lightroom.
Since then whenever I open LR it always loads that catalog.
I marked this as the correct answer, however the real answer was that I forgot that the catalog default setting was in the General tab and not the catalog setting tab. If you have any pull with the LR engineers, they should either put it in the catalog settings tab or duplicate it there.
Your answer pointed me to the real setting I needed to change.