drcachesim offline fails on fork child with error about file creation

I first hit this tracing SPEC2017 perlbench. I can reproduce it easily:

$ bin64/drrun -t drcachesim -offline -- suite/tests/bin/linux.fork
parent is running under DynamoRIO
<-- parent 564285 forked child 564286 -->
Failed to create new thread file for window ./drmemtrace.linux.fork.564286.0539.dir/raw/drmemtrace.linux.fork.564286.7156.raw.lz4