GPT-4 loses the conversation thread

In the last few days we’ve noticed that GPT-4 is not retaining the conversation thread. Using the API (in Azure) it is trying to answer the context with the previous question in the conversation. In the browser we are getting an error of ‘Conversation key not found’ when asking a follow on question. (We understand the inherent challenges of prompt size etc and know that this is not the cause of this issue). GPT-3.5-turbo does not exhibit these issues.

Has anyone else experienced this?