Lossless source does not mean identical containers
FLAC compresses audio without discarding the encoded sample information. WAV commonly stores uncompressed PCM samples. When the FLAC is decoded to a compatible PCM representation, the audio samples can be preserved even though the file structure and size change substantially.
Check the source sample rate, bit depth, and channel count before choosing a WAV workflow. A conversion that changes sample format is not the same as merely unpacking the compressed data. Keep the original FLAC as a compact archival copy and treat WAV as a working or handoff version when a particular application requires it.