Hello,
Is this issue occurying only on 1 of this client machine OR are there more 64 bit machines which are having similar issue?
Could you please provide us a screenshot of the SEP client when you start your 64 bit client machine?
If this happening on a number of 64 bit client machines, are these then all from same group in SEPM?
Since these machines are managed client, check if the Tamper Protection is enabled.
If Tamper Protection is enabled on the Client, it will not allow the service mode to be changed to automatic mode.
Tamper Protection Settings can be enabled and disabled from both SEPM and SEP Client.
SEPM > Clients > Group > Policies > Location Specific Settings : General Settings > Tamper Protection
SEP > Change Settings > Client Management > Tamper Protection.
Hope that helps!!