Partial merges of stack PRs in the UI produces unsigned commits

[ausp]

Merging an earlier PR in the stack via UI produces unsigned commits in the later PR(s) as a result of a rebase.

This might be due to a web config issue, but if so I've been unable to find it.

Before merging, commits in question, signed:

Before merging, state of play of earlier PR:

Before merging, state of play of later PR:

Post-merge, commits are rewritten and unsigned:

Our repos require commits to be signed, and this is interfering with CICD.

A local run of gh stack sync has been seen to resolve this, but it is a fair amount of overhead to perform each time we want a partial merge for review purposes.