OpenBSD's
urwt(4) driver is relatively new, it was introduced in 4.5.. originally developed for FreeBSD by Weongyo Jeong in 2009.
The original driver only supported the RTL8187L, but if you read the CVS commit history.. support for the RTL8187B was introduced just before the release of 4.6.
martynas@ is the current maintainer of the urtw(4) for OpenBSD, he also did the original port from FreeBSD.
Contacting him directly may be the best way to diagnose issues with your device, alternatively you may try submitting a bug report via
sendbug(1).