Changing the Makefile for the TLS module to use OpenSSL 1.0 fixes the problem, that at least provides us with a workaround.
Just to be sure, I've repeated the process of making the TLS module using OpenSSL 1.1.1b and the problem returns, if I then clean and make the TLS module using OpenSSL 1.0.2r the problem is fixed.
Thank you to all for your assistance.