Single Bundle Image (SBI) upgrade to 12.1-53.12 will upgrade the Platform and will move the LSI Firmware to the latest 23.34.0-0019. This latest LSI Firmware version ( 23.34.0-0019) has a fix for the SDX hung issue. Thus, install 12.1 build 53.12 using Single Bundle Upgrade Image again and ensure if the LSI Firmware version has upgraded to 23.34.0-0019.
After the Single Bundle upgrade, you can verify the LSI firmware either from the XenServer console using the below command or by collecting the support bundle of XenServer of SDX appliance.
/opt/MegaRAID/MegaCli/MegaCli -AdpAllinfo -aAll