Hi Paul,
I apologize for missing out your initial question "Which "bit" is this referring to. There is no description of any initialization operation in either of these registers."
The "bit" mentioned in ECCCTRL1, is register 0xFFCFB100
while, the "bit" mentioned in ECCCTRL2, is register 0xFFCFB104
Are you configuring the HPS external memory interface (EMIF) to be 16 bits wide with error checking and correction (ECC) enabled? If so, please refer our Intel® Arria® 10 SX Device Errata and Design Recommendations below:
https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/es/es-1053.pdf#page=9
Regards.