you are looking at the strace output in real time

Not really. - I would start such program tracing only after a personal delay to notice that data processing for the function “QFileDialog::getOpenFileName” might take longer than what I would usually expect.
(I might be more patient under other circumstances.)

what you would see is the last line of output would "hang"

This did not happen. The dialogue software might be still busy with other stuff.
Additional activities might distract then from temporary technical difficulties.

just as you went into a "delay", and that would be your indication as to why....

I found questionable software behaviour also for other components in the suggested way so that corresponding clarification requests and bug reports were published.