I posted with a similar problem a month ago. As stated above, you need to hold the strop tight, with light touch, and no sag during stropping. This will avoid rounding the edge. To make sure I wasn't rounding the edge, a few times, I laid the strop on a table and stropped there. (The table can't sag!) When things improved there, I returned to the hanging strop, and the dulling problem after one shave didn't return.

As an aside, I am increasingly of the belief that I wasn't stropping enough before. With a microscope to view the edge, and with many shave tests, I am slowly increasing the number of stropping passes I use.