Here's the full backtrace, I printed this out at a few different occurrences of the log and it appears they all have the same backtrace, * thread #51, name = 'com.apple.coremedia.JVTlib', stop reason = breakpoint 2.3   * frame #0: 0x00000001800c4878 libsystem_c.dylib`fprintf     frame #1: 0x00000001c9462d8c H264SW.videocodec`set_affinity_tag + 108     frame #2: 0x00000001c94626a8 H264SW.videocodec`ThreadedBatchSlaveMain + 48     frame #3: 0x00000001c99ce68c libsystem_pthread.dylib`_pthread_start + 116

Alright looking at the backtrace for this I believe this log is coming from Apple, specifically com.apple.coremedia.JVTlib, Please let me know if I can provide a different output format libsystem_c.dylib`fprintf: ->  0x1800c4878 <+0>:   sub    sp, sp, #0x40             ; =0x40      0x1800c487c <+4>:   stp    x22, x21, [sp, #0x10]     0x1800c4880 <+8>:   stp    x20, x19, [sp, #0x20]     0x1800c4884 <+12>:  stp    x29, x30, [sp, #0x30]     0x1800c4888 <+16>:  add    x29, sp, #0x30            ; =0x30      0x1800c488c <+20>:  mov    x19, x1     0x1800c4890 <+24>:  mov    x20, x0     0x1800c4894 <+28>:  add    x8, x29, #0x10            ; =0x10      0x1800c4898 <+32>:  str    x8, [sp, #0x8]     0x1800c489c <+36>:  adrp   x8, 360430     0x1800c48a0 <+40>:  ldr    x0, [x8, #0x7d8]     0x1800c48a4 <+44>:  adrp   x21, 360430     0x1800c48a8 <+48>:  add    x21, x21, #0x210          ; =0x210      0x1800c48ac <+52>:  cmn    x0, #0x1                  ; =0x1      0x1800c48b0 <+56>:  b.eq   0x1800c48c0               ; <+72>     0x1800c48b4 <+60>:  bl     0x1800ff3fc               ; symbol stub for: pthread_getspecific     0x1800c48b8 <+64>:  cmp    x0, #0x0                  ; =0x0      0x1800c48bc <+68>:  csel   x21, x21, x0, eq     0x1800c48c0 <+72>:  ldr    x3, [sp, #0x8]     0x1800c48c4 <+76>:  mov    x0, x20     0x1800c48c8 <+80>:  mov    x1, x21     0x1800c48cc <+84>:  mov    x2, x19     0x1800c48d0 <+88>:  bl     0x1800caf24               ; vfprintf_l     0x1800c48d4 <+92>:  ldp    x29, x30, [sp, #0x30]     0x1800c48d8 <+96>:  ldp    x20, x19, [sp, #0x20]     0x1800c48dc <+100>: ldp    x22, x21, [sp, #0x10]     0x1800c48e0 <+104>: add    sp, sp, #0x40             ; =0x40      0x1800c48e4 <+108>: ret

Thanks Quinn, This was super helpful, I confirmed via the code you linked that the log messages are coming from stderr.