Question: Which of the "Relationship" TOOLS are you referring to?
There are two (x2) that I know of ...
"View My Relationship" ... to the individual/person one is working on.
I just double checked ...
It shows my relationship through, both, "Maternal"; and, "Paternal', lines ...
Plus, I even checked with my Wife's Grandparents - it worked just fine
[ ie. Wife's Grandmother/Father - on, both, "Maternal"; and, "Paternal', lines ]
Seems to be working just fine.
"Enable Relationship Viewing" ... between Yourself; &, ANOTHER User/Patron
That I have not tested ...
That said ...
Here is a "Knowledge Article" in 'FamilySearch':
How do I view my relationship to a person in Family Tree?
Although, not specifically indicated, there is no mention that such only looks at one specific line (eg. either, "Maternal"; or, "Paternal").
My reading is that it explores, both, "Maternal"; or, "Paternal", lines.
As, I indicated, I have not tested such; so, I could be wrong ...
Just my thoughts.
Just depends who you are linking to.
also note the viewer will only show the first relationship it finds.
you could ask similar questions about various charts and forms
to a certain extent we trust it is working unless we have proof it is not.
I have no reason to believe it is not checking both sides of the parentage.
if you have a specific reason you think it is not working let us know that specific reason.