Hi,
The problem is quite expectable, because you don´t comply with SPI standards. You "short circuit" two outputs (SST25 and ISO7441 on signal SDI_SDI)
This way you can easily kill one or both devices.
--> When a device id DESELECTED the MISO line needs to be HIGH IMPEDANCE.
The SST25.. works correctly on this.
But: For this the ISO7741 has an EN input. This is why they designed it in. Just use it!
***
Btw:
Why does /WP line have two pull up resistors?
Klaus