Replies rendering as regular messages in some rooms

[Evidlo]

I have a recently-created room where replies from Slack users are not being rendered in Gomuks correctly (but work OK in Element):

Below is the message source for one of these messages:

{
  "content": {
    "body": "Yeah - the original version had all booleans and then I never renamed the variables.",
    "format": "org.matrix.custom.html",
    "formatted_body": "Yeah - the original version had all booleans and then I never renamed the variables.",
    "m.relates_to": {
      "event_id": "$dQX5jVYj03dVHpkvFQ-MX3eQmowcKVvD-keTmc1o-HU",
      "is_falling_back": true,
      "m.in_reply_to": {
        "event_id": "$rLH5HQjlQUnyCAd2yIylQQKQMBRgyZP23Et8yK77wTw"
      },
      "rel_type": "m.thread"
    },
    "msgtype": "m.text"
  },
  "origin_server_ts": 1709653658684,
  "room_id": "!oiCQlJTrnxLRMOYrbg:matrix.org",
  "sender": "@_slack_carruthers-workspace_U04FU7FCZDF:matrix.org",
  "type": "m.room.message",
  "unsigned": {
    "age": 7683505
  },
  "event_id": "$UUpZhtPQIlzmKMgMnm64c6CtLFHq6PM63tIV8_qSwvA",
  "user_id": "@_slack_carruthers-workspace_U04FU7FCZDF:matrix.org",
  "age": 7683216
}