quartus programmer says EPCS is blank when it is not

I'm seeing something very strange with a previously working Cyclone IV GX design.

I used Quartus programmer to erase an EPCS64 attached via AS programming pins to a C4GX. (Factory default enhanced SFL image sof loaded in C4GX). The erase operation completed without error. I also did a successful blank check. However after cycling power on the board the C4GX configured with the previously programmed image! This procedure has worked i.e. actually erased the device until recently. I repeated the erase/blank-check several times with the same result each time. Quartus programmer never reported any errors, but the device was never erased.

I tried 11.1 standalone programmer and programmer within Quartus - same result.

I then switched to the Nios programmer and used my C4GX image that contains an EPCS controller in the Nios system. I successfully used nios2-flash-programmer --epcs --erase-all to erase the flash. This programmer confirmed that the device was not previously erased. Now the C4GX doesn't configure after a power cycle.

Has anyone seen this behavior using Quartus programmer, or otherwise from an EPCS device?