Jamie - another example:
It happened to me Jan 23, 2021. I added a child for Hall-50530 with first name Annie, LNAB Reynolds, CLN Ellis, Female, Birth 1894-11-15, and Death 1977-01-13 and checked the option that her father was Reynolds-21399.
I don't recall what (if any) showed up as possible matches, but I can confirm that if there were any, I checked the list carefully and saw no potential matches already here.
After adding the profile (Reynolds-21570), I immediately proceeded to add seven sources and write a biography based on these records. Finally, I was ready to add her husband but a possible match was shown for him and it turned out to be him, plus he already had a wife.
The already existing profile Reynolds-5151 was created April 3, 2014 and is very minimal - only WikiTree's boilerplate for biography and sources. No parents are shown and the only connections are to husband and one of their five children. First name is Annie, Middle Name is Mae, LNAB and CLN are both Reynolds, and Birth Date is about 1895.
It seems like this should have shown up as a match when I was adding what is now a duplicate. I proposed the merge as soon as I found the duplicate and it is still awaiting what will probably end up as default approval because the manager of the original one was last active nearly 2 years ago.