Harper isn't active in the GitHub title editor

I noticed that I've had a spello in the title of one of my PRs where I wrote "unweildy" instead of "unwieldy". But when I click to edit it, it's not flagged.

But as you can see from this screenshot where I was using Harper Glasses, it is recognized as a mistake and it is active on GitHub. Just not on this field for some reason:

No file or test is named in the report. Start by reproducing the issue in a GitHub pull-request title editor, then trace the Harper integration responsible for checking that field. Done means the misspelling “unweildy” is flagged there as it is when Harper Glasses is active.

Written by the indexing model from the issue text.