I unfortunately misunderstood how ${FLUTTER_LOGS_DIR} works, and have been overwriting my own outputs 😢:

1. Any task in the same builder writes to the same directory

   If task A writes foo.txt, and task B writes foo.txt, then task A's foo.txt is silently overwritten.

2. Re-runs (i.e. due to failures or timeouts) have the same property

   If task A (run 1) fails, a successful task A (run 1) will overwrite the logs of the failed entry.

This is all made extra confusing by the fact the UI shows multiple log uploads, as if they might be different:

Special thanks to @keyonghan for explaining this all to me here on Discord.