PROBLEM RESOLVED WITH AFTER MARKET EAR TIPS.
Further to my previous theories that the problem is caused by a flaw in the the Apple Ear Tip design - most likely the mesh surfaces trapping moisture, I decided to try an after market Ear tip. I purchased the Comply ‘MAX Truegrip’ Memory Foam Ear Tips. I took these on a 3hr+ bike ride and no issues. I then wore them for the remainder of the day while building a fence in hot sun. No issues at all. TBH they also have better sound quality and have a more comfortable fit to the Apple Version. They are quite pricy however at $51 NZ ($30 US) for 3 sets.
I am very disappointed that Apple has not addressed the issue, and don’t seem to be owning it at all. I have had no further response from them and wasted countless hours and money trying to resolve this!