If you are seeing 503 errors in your function app, you can check the function app's metrics and logs to investigate what may be causing the high load. You can see these metrics under the Monitoring tab of the function in the portal.
From the context of you question though, the 503s could be occurring on/at the front-end worker. And since the Consumption Plan automatically scales out, there may be an issue during that operation if you're seeing it consistently. One option to consider is scaling out the function app on a higher tier to handle the load and see if that resolves the issue. If it does, you can at least see what HTTP triggers are causing high load through metrics, address them, and the try to downgrade back to Consumption plan.
If you've already done that, then we'll need to work more closely with you to investigate this issue.
EDIT 2023 May 12
Currently, Function Apps doesn't support HTTP log capture like web apps. However, the lack of logs from the function instances and timeout/service unavailable errors received by the client sending the HTTP request has good correlation to these types of failures.