I found that ExtPrivateKey.apply does not provide any seed bit-length checks.

However, minimum and maximum bit-length constraints are defined in BIP32.

Generate a seed byte sequence S of a chosen length (between 128 and 512 bits; 256 bits is advised) from a (P)RNG.

I wonder whether there is a specific reason for this not being enforced.

This was discovered using the bitcoinfuzz tool, leveraging the Bitcoin-S master key generation fuzzing target implemented by @guptapratykshh. Thanks!