Bir Sur partition gets deleted

Hi,

I have 2 partitions in iMac (Catalina & Big Sur). Recently I have switched from Big Sur to Catalina from startup disk, later I tried to switch to Big Sur but it was not showing in startup disk. Tried to switch OS while booting, but Big Sur partition did not show up. When system restarted and I checked disk utility there too Big Sur is not listed. Anyone has faced this ?

Any help would be really appreciated.
Bir Sur partition gets deleted
 
 
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