FSSO works based on the logon events. Usually MAC PC do not generate a Windows logon event in the AD server. So FSSO for MAC may not work properly. If you are able to view the event logs in the AD server, you can try with polling mode and select "windows security event logs"
If the event logs are not seen, as a workaround you can try using NTLM option. This is a browser based authentication that will require users to logon a second time via the browser. The non-windows machines will get prompt to enter the AD login details. Below is the command for enabling NTLM:
config firewall policy
set ntlm enable
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