Yeah, this happens a lot, sometimes one of the custom planets in a custom star is missing, and as a result you will see the debug message "fail to place planet". I believe it has to do with all the planets has to be in the same sector, and the normal positioning of that last planet would have put it out of the sector. However, I must admit, this is the first time I've seen a situation where the UP picks this planet.
It's definitely a bug with the UP, which, hopefully will be looked into more closely for the 1.2 patch (a lot of the resolutions are erroneous). Right now, I don't think you can do anything about it, you're just unlucky (just be glad that nothing bad happened

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