Unprocessable error when submitting pull request to GitHub

[francois]

Using Sapling 0.2.20230523-092610+f12b7eee, I started getting errors while submitting pull requests to GitHub:

╰─○ sl pr submit
pushing 1 to https://github.com/Inspectify/marketplace
abort: unexpected error when trying to merge 49d2804b02e3198cc3bd0d775852c71320b04957 into sapling-pr-archive-francois: {
 "data": {
  "mergeBranch": null
 },
 "errors": [
  {
   "type": "UNPROCESSABLE",
   "path": [
    "mergeBranch"
   ],
   "locations": [
    {
     "line": 3,
     "column": 3
    }
   ],
   "message": "Failed to merge: \"Head does not exist\""
  }
 ],
 "extensions": {
  "warnings": [
   {
    "type": "DEPRECATION",
    "message": "The id MDEwOlJlcG9zaXRvcnkyNDcxNzIxMzM= is deprecated. Update your cache to use the next_global_id from the data payload.",
    "data": {
     "legacy_global_id": "MDEwOlJlcG9zaXRvcnkyNDcxNzIxMzM=",
     "next_global_id": "R_kgDODruMJQ"
    },
    "link": "https://docs.github.com"
   }
  ]
 }
}

The remote branch was updated with the latest code though, and CI kicks in normally.

[francois]

This is inconsistent: I just submitted a new PR and didn't get that error. I received the error two or three times yesterday.