When we temporarily switched PHP7 to 0%, we noticed that socket errors decreased. Socket errors kept increasing as we were pushing more and more traffic to PHP7.2, specifically the 'tcp/attemptfails' metric
After researching, we found out that the kernel increases this counter when
a) a tcp packet has both RST and SYN flags set (not the case here)
b) a socket is left in a SYN_SENT or SYN_RECV state
in our case
# netstat -s 4224373 failed connection attempts