It’s important to know that the list on the Shared tab page is restricted to Ancestry’s chosen CM threshold. The way they put it is that they only show you “fourth-cousins-and-closer”. That translates into above 20 cM.

...However, person A’s sibling, person B, also matches me below 20 cM, but I can’t see that shared match with person A because my shared match with person B is below 20 cM. However, checking my match list for person B’s name shows that they are a match to me. However, there is no way to know that I match person B in common with person A.

 

Then, checking another family member, like an aunt, for example, I see that person A and person B both match her as well, probably also on segments below 20 cM so she can’t see them on her shared match list either, nor can I see either of those matches, person A or person B on my shared match list with my aunt.

 

Reaching out to matches below 20 cM and asking if they have other family members you can check, by name, to see if they are on your match list is important. Many people don’t realize shared matches below 20 cM aren’t shown at Ancestry.

 Looking through my matches, there are no 5th-8th cousins in shared matches i.e. below 20 centimorgans, can't believe I haven't noticed this before! 

Correct - shared matches are only people you share at least 20 cM with and the other person also does.

I have a third cousin, who I share with 3 others. If I click on the 3rd cousin's shared matches it shows a 4 to 6 cousin - the two 5 to 8 cousins do not show at all. If I look at one of the 5 to 8 cousins it shows the 3rd and 4 to 6 cousins, but not the other 5 to 8 cousin. So, only 4 to 6 cousins, or closer, show in shared matches. This has been the case for me since April when I got my results.