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lightning MPP pay splits into too many subpayments

I have an outbound channel with an BTC to Lightning exchange worth 2.000.000 satoshi, with enough capacity. I try to swap my 2.000.000 satoshi for the same amount, but on-chain. Lightningd splits the payment into too many subpayments, and eventually fails, even though the destination is only one hop away, and has enough capacity.

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Without any help

And surprisingly ... legacypay same_bolt_11 5 minutes later worked.