reports: unable to generate PDF/CSV when there are a large number of searchable fields

It appears that if an installation has a large number of custom fields that are flagged as searchable, you will reach a point where reports may not generate PDF/CSV exports.

I though the issue was that we store ALL of the report's criteria/filter fields as a serialized array in the form_values column. That column is a TEXT type field, and consequently if our serialized value exceeds the max size, it is truncated, which would cause problems. However, increasing the form_values column type doesn't fix the issue. So there is something else going on as well, but I've been unable to determine what.

FYI –
It definitely relates to the volume of custom fields. If I disable a few groups the PDF/CSV generation will start working again.