Stick thin pieces of double-sided tape in the 4x5 holder.
One batch cut too small, another batch cut too large. Does not sound good to me.
I've been using Harman DPP for over a year, in both 4x5 and 8x10 sizes, and haven't seen this problem. However, early on in their US distribution they were selling 4x5 that was exactly the print size, not the sheet film size, necessitating it to be cut down to fit into sheet film holders. I wonder if this pack was the sheet film size that had then been mistakenly cut down a second time, making it too small.
It's a great product, however, and I would not hesitate to keep using it.
The Harman Express site indicates that Direct Positive Paper item #1171158 is nominally 10.2cm X 12.7cm (4" X 5" Imperial) but actually measures 9.96cm X 12.5cm. This is the size that goes into standard 4x5 film holders. There used to be (still is?) a genuine 4"x5" DP paper that is a paper size, not a film size, and needs trimming to go into a film holder.