Yes, this is happening to me too - incredibly frustrating. I haven't bothered to confirm if this is the case: that the photos are actually getting their metadata merged and the wrong date is being used, or if Photos is just choosing the photo to keep based on a set of ordered rules (e.g. keep photo with larger resolution > keep photo with higher DPI > keep photo with earlier capture date > keep photo with earlier file creation date > etc.). I bought a 3rd party application called Photosweeper to try and do a better job at removing duplicates and it uses rules such as this, but has its own (few) issues.
In my case, I have hundreds of duplicates due to exporting old photos off Facebook and Google Photos that I had (re)uploaded in different years. Problem is they had stripped the original EXIF data so the original capture dates are lost forever. The next best thing is using the dates on the Facebook versions because it's likely they were uploaded closest to the capture dates.
To the original topic, I don't know why Photos would not just merge capture dates and use the earliest one. Surely the far more common case is you have photos that have a date that is later or much later than the date they were taken. Very rarely would you have an incorrect capture date that predates when the photo was taken. Or just let us choose to iteratively go through merge groups and select which metadata to keep like a DIFF/MERGE tool.