During mutual close negotiation, Eclair sometimes does not send the last closing_signed msg.
Specifically I am talking about this part of BOLT-02:

https://github.com/lightningnetwork/lightning-rfc/blob/b201efe0546120c14bf154ce5f4e18da7243fe7a/02-peer-protocol.md#requirements-6

Closing Negotiation: closing_signed
Requirements
The receiving node:

• if the receiver agrees with the fee:
  • SHOULD reply with a closing_signed with the same fee_satoshis value.

Consider a channel between Alice and Bob, with Bob running Eclair.
If Alice sends a closing_signed with a fee_satoshis Bob agrees with,
I've found that

• sometimes Bob replies with a closing_signed with the same fee_satoshis value,
• other times Bob does not reply and just broadcasts the on-chain tx.

I am looking at this part of the code:

Consider scenario1:

• Alice sends closing_signed with fee_satoshis = 504,
• Bob sends closing_signed with fee_satoshis = 500,
• Alice sends closing_signed with fee_satoshis = 503,
• Now I think Bob will calculate nextClosingFee to be (500 + 503) // 4 * 2 == 500,
  which matches what he sent last, and will not reply, just broadcast the tx (with fee_satoshis = 503).

Ideally, Bob should send a final closing_signed with fee_satoshis = 503,
as this is what BOLT-02 says he should do.

Consider scenario2:

• Alice sends closing_signed with fee_satoshis = 508,
• Bob sends closing_signed with fee_satoshis = 500,
• Alice sends closing_signed with fee_satoshis = 502,
• Now I think Bob will calculate nextClosingFee to be (500 + 504) // 4 * 2 == 502,
  so Bob enters the "we have converged!" branch,
  replies with a closing_signed with fee_satoshis = 502,
  and broadcast the tx (with fee_satoshis = 502)

(so scenario2 works out as expected)

So my issue is that - like the comment in the code suggests - due to rounding errors, to an outside observer Eclair behaves probabilistically: sometimes it sends the last closing_signed messages, sometimes it does not.