Unfortunately the issue remains, but at least now we're rid of the shared memory issue:

RT: connected to control stream 127.0.0.1
RT: version 23 Jan 2020: ROIs_and_data
RT: init RT input via control channel
RT: waiting for data stream to become good.
RT: receiving image metadata=164 bytes
RT TIME (starting to receive realtime data): 2128 sec, 434 ms (0 ms passed)
## PARSE RT Detrend: RT Options mode 0x0 polort -1 fwhm 0
RT: finish channel 01 with 1 bricks completed.
RT: cpu time = 0.02 elapsed time = 0.39
RT: sending dataset rt.__001+orig with 1 bricks
to AFNI[A], session /Users/tevesjb/Downloads/jt_rt/rt_try/Realtime/

Fatal Signal 11 (SIGSEGV) received
AFNI_autorange_label
AFNI_reset_func_range
AFNI_setup_viewing
AFNI_initialize_view
AFNI:main
Bottom of Debug Stack
** AFNI version = AFNI_21.0.21 Compile date = Mar 24 2021
** [[Precompiled binary macos_10.12_local: Mar 24 2021]]
** Program Death **
** If you report this crash to the AFNI message board,
** please copy the error messages EXACTLY, and give
** the command line you used to run the program, and
** any other information needed to repeat the problem.
** You may later be asked to upload data to help debug.
** Crash log is appended to file /Users/tevesjb/.afni.crashlog