I've tried running cpuminer-opt 3.7.6 with lyra2z on a 48 CPU system - unfortunately, only 32 CPUs are used.
The program says "48 miner threads started, using 'lyra2z' algorithm", with or without -t 48 option.
cpuminer-opt 3.7.3 behaves correctly and runs on all 48 cores.
Can you clarify? Does it start 48 threads and run 2 on some cores? Posting the program output would help.
What kind of CPU architecture do you have, multisocket?
Edit: also what OS and please post command line. I always need this, I shouldn't have to ask.