The "with_bootloader" export which happens here...

https://github.com/arduino/arduino-builder/blob/29cf4c8b1473b4d0ba429c67ee497e2e061bd2fc/merge_sketch_with_bootloader.go

has some issues.

1. There is no output or even logging that it happened, so we can't for example see which bootloader it picked easily, how much it threw away, and that sort of thing.

2. The attached zip contains a compiled sketch .hex, the bootloader which it (in theory) selected, and the merged hex. The merged hex however contains the sketch hex, and two lines only from the bootloader hex - you will see clearly it can't be right because the bootloader is 9.9k, and the "merged" file which should contain both the sketch and bootloader is only 640 bytes.