getblockheader produces empty signblock_witness

[jamesdorfman]

The getblockheader RPC returns two fields pertaining to the signblock_witness:

• signblock_witness_asm
• signblock_witness_hex

Actual behavior
These are both currently empty.

Expected behavior
I expected these fields to contain a signblock_witness that corresponds to the signblockscript (which is not empty).

To reproduce
On the liquid network, you can currently reproduce this issue with

$ elements-cli getblockheader $(elements-cli getbestblockhash)

Example:

$ elements-cli getblockheader 1f8b6094977bfc13164752923a01d5a0b628de160c37d7c66ad923620c2e0bcb
{
  "hash": "1f8b6094977bfc13164752923a01d5a0b628de160c37d7c66ad923620c2e0bcb",
  "confirmations": 28,
  "height": 1748299,
  "version": 536870912,
  "versionHex": "20000000",
  "merkleroot": "b325c89c57ba6d2df95bf9433c903f8f020b3f8e24ec49d01181b1889461a586",
  "time": 1647445868,
  "mediantime": 1647445568,
  "signblock_witness_asm": "",
  "signblock_witness_hex": "",
  ...
}

System information
I am running macOS Monterey v12.1.