make.conf(5) contains the following statement about MALLOC_PRODUCTION:
(bool) Set this to disable assertions and statistics
gathering in malloc(3). It also defaults the A and J
runtime options to off. Disabled by default on -CURRENT.
I believe the A and J options were from the old malloc implementation (phkmalloc?), while FreeBSD has used jemalloc for many years, which uses a different set of options.