@Richard Haigh Thanks for reaching out.
Based on the information that you have provided it is not normal for Azure Functions to experience such blips in connectivity after continuous operation until and unless any configuration changed at the key vault, network or function end and there could be several reasons why this is happening.
Regarding your question about the "turn it off and on again" approach, while it may temporarily resolve the issue, it is not a legitimate long-term solution. It is important to identify the root cause of the issue and address it accordingly.
For your second point this could confirm post reviewing the logs. It is difficult to determine the exact cause of the issue without further investigation. I recommend checking the network and firewall settings, validate operation logs of your function app, key vault if there was any update operation was performed and in case if you are not able to find the root cause then this would need one on one support to review the logs if we can find the root cause. I suggest you to create a support ticket to assist you further. In case if you don't have the support plan then please let me know.