Sorry to hear you are having troubles after updating WP. It is possible that a change in WP code is not compatible with such an old version of ACF, however, I doubt this is the issue.
With such a large amount of code dumped above, I am not able to help by pointing out any issues. Perhaps you need to take a step back from the code, and start fresh adding it in line by line until you find the exact problem.
You mention that there is a filter involved which is causing the issue? What is this filter code?
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