I just experienced what I think may (or may not) be the same problem when copying from 15 and pasting into 19 (but the different versions won't matter).
The walls came across but the doors didn't in plan.
They were there in 3D.
But what I noticed is the wall was "Projected with Overhead" plan display.
This is fine but my Floor Plan Cut Plane height was different in the 2 files - lower in the original (so all looked fine there) but higher in the second (paste) file.
The FPCP height was above the top of the wall so all I could see was the wall outline and no doors.
As soon as I changed it back to "Symbolic" the doors appeared.
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11