It can be pushtx'd to you too, I think, when a peer invs it to you despite feefilter and then you fetch it. Violating feefilter was at least on prior versions not a banscore 100 event.

I'm not sure if it is still possible to send someone a tx straight without getdata (used to be though) which will put it in your mempool but it won't be relayed.

Is your mempool explorer publicly accessible? Maybe someone used the "Broadcast Transaction" (https://mempool.space/tx/push) feature?

No CPFP:

$ bitcoin-cli getmempoolentry c17e5f9f353fbb70128f6455aa7b79b02a74c7a9d34b10d1da5009e5e9de7701
{
  "vsize": 177,
  "weight": 706,
  "time": 1789605732,
  "height": 967348,
  "descendantcount": 1,
  "descendantsize": 177,
  "ancestorcount": 1,
  "ancestorsize": 177,
  "wtxid": "546ba5824e5d2ed5eb7647255d818fb47d2f6b7b2977febf0547abe51bb9e45c",
  "chunkweight": 706,
  "fees": {
    "base": 0.00000000,
    "modified": 0.00000000,
    "ancestor": 0.00000000,
    "descendant": 0.00000000,
    "chunk": 0.00000000
  },
  "depends": [
  ],
  "spentby": [
  ],
  "bip125-replaceable": false,
  "unbroadcast": false
}

So I thought maybe there was a small reorg and its block got reorged out. But according to fork.observer, the last reorg was at 967143 which was ~2 days ago and does not line up with the tx appearing 18hrs ago.