FILE_ADDED events getting lost

I'm using a mix of Nikon D5300s and D5600s, and we're triggering the
focus and shutter externally using hardware (through the MC-DC2 port),
and then I'm using gp_camera_wait_for_event() to spot a
GP_EVENT_FILE_ADDED event, so I can know when to grab it.

I was using the timeout support in that call, but I wasn't seeing a
correlation between real time and the stated milliseconds I was
passing to gp_camera_wait_for_event(), so I tried passing 0ms, 1ms,
and 10ms timeouts, and then I just polled that for 10 real seconds by
checking clock_gettime(CLOCK_REALTIME) until I got 10 real seconds
passed since the start of the loop.

I'm not sure if it was that change or something else, but I am
definitely no longer getting reliable GP_EVENT_FILE_ADDED events, even
when I confirm that the file did show up on the internal SD card.

For some reason, instead, I'll get three GP_EVENT_UNKNOWN's, and
sometimes, oddly, the next time I restart my code using
gp_camera_init() and friends, when I go through and discard any events
that might have been queued up by initialization, I'll sometimes see a
GP_EVENT_FILE_ADDED, but even more maddeningly, not always.

Resetting the camera power will usually allow it to work for one shot
with the GP_EVENT_FILE_ADDED event showing up properly, and then on
subsequent shots, it'll get back in that state where it gives me those
GP_EVENT_UNKNOWN's, never to return until the camera is power-cycled.
Note that restarting my process using libgphoto2 has no bearing on
recovering from this state.

Also, does the fact that the timeouts don't seem to track real-time
sound familiar to you?  Or am I a weirdo for seeing that?

What I'm really hoping is that these symptoms sound familiar to you,
and you suggest what I might next try to further isolate what's
happening.

I'm on the Debian package libgphoto2-dev 2.5.22-3 on a Raspberry Pi 3
running Raspbian Buster.

I'm kind of at my wit's end how to further isolate this.  If you could
suggest any strategies to help track this down, I'd love suggestions!
Thank you!

       =-ddt->