Ok, I resolved this problem by reverting back to 188.8.131.52. As soon as the phone finished booting with the 184.108.40.206 firmware, I heard the CS540 base click active. I pushed the button on the headset to deactivate it and I knew it was going to work before i even picked it up to test.
This does not explain why the EHS previously worked with 220.127.116.11.
After confirming everything working correctly, I upgraded to 18.104.22.168. The headset no longer worked correctly. I downgraded back to 22.214.171.124 and it worked correctly again.
This is a problem with the new firmware for the phone and obviously not a problem with the 126.96.36.199 EHS firmware.
Email sent to yealink support. Hopefully this will be resolved in future firmware.