Why is merging limited to one direction which is not the preferred direction?

Generally speaking, when merging, it is preferred to merge a newer profile into an older, existing profile to retain the years of change history. However, sometimes the system will not allow the merge to occur in the preferred direction. I am working with John Gwynne L8PF-QX8 who was created in 2012 and for whom there are four sources. I am trying to merge John Gwynn GM4P-GR5 who was created in 2018 into the first man's profile. The second John has no sources, and both men are connected to the same wife, Margaret. Why does the system force the merge the wrong way in this scenario?
Comentarios
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Long time ago I heard a presentation in which this was discussed and there were various reasons such as number of source, number of notes, and other things which due to the way a merge is carried out would cause the process to crash if merged in one direction but not the other. I don't see anything particularly complex on either of these profiles. So there must be something the underlying data of the profiles that is not displayed that is triggering this.
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