[bug]: sending update_fail_malformed_htlc with error code 4103 (aka temporary_channel_failure)?

[rustyrussell]

Background

Report that Bitrefill is sending these, and CLN is issuing a warning and closing the connection:

ElementsProject/lightning#6425

Seems weird, but pretty sure they're LND?

Steps to reproduce

It obviously doesn't happen very often: the result of this would be a channel closure when we eventually get sick of the HTLC and it times out.

Posting here in case it's a known issue or been reported before?

[Roasbeef]

Perhaps there's an onion replay happening somewhere in the route? See this issue for an example: #5792 (comment)

There's a known bug where we don't properly transform the error internally before sending out on the wire: #5443 (comment). Looks like this fell through the cracks shortly after the realization in that issue, will reprio....