Capture a sniffer trace and see what's going on.
When you do not specify the group, the CSM assumes this is the default sticky group 0 which is stickyness based on source ip.
Also, verify if the problem exist with all types of browser or just IE for example.
IE is well-known for not working very will with ssl stickyness because it keeps changing the sslid.
Regards,
Gilles.
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