--size-prefixed generates empty annotated file

[BANOnotIT]

Running annotation with --size-prefixed generates almost empty file

Reproduction

1. Get Test.py, sample.py, and test.fbs from this gist.
2. install flatbuffers from pip
3. run python sample.py
4. run flatc --size-prefixed --annotated test.fbs -- prefixed_sample.dat
5. run flatc --annotated test.fbs -- sample.dat

Expected

prefixed_sample.afb file should contain similar content to sample.afb

Actual result

// Annotated Flatbuffer Binary
//
// Schema file: test.fbs
// Binary file: prefixed_sample.dat

flatc version 25.12.19