This is one of my pet peeves of FTM software.
It also exhibits this behavior with regards to persons you may designate as having a step-relationship with (by using the Relationship button at the person page and designating someone a step-parent of child.
Note that one can alter the FTM behavior in ANCESTRAL reports by being sure to designate the "blood" parents as the "preferred parents". But this doesn't help with DESCENDANT reports.
What I am beginning to do with my surname study is to put persons "adopted-IN" to my family twice. One a dummy marker person that says that person was adopted-in and (usually) he has been set up with a parent of "Surname Adopted-in" to designate a new blood line within that surname was started. For women adopted in, I may just show a marriage and children and not take her further, because her descendants don't continue either the name or the blood of any ancestor of her adoptive parents.
I have only done this for my surname study. For my other 125 New England lines, adoptees are sprinkled through their families. I have a custom fact setup to show the line of descent of a person. For adoptees, I try to remember to put "Smith - ADOPTED-IN LINE".
People "adopted-OUT" of my surname study don't present the same problem, as they are truly blood descendants of their blood parents, and will properly show as blood descendant with a name change. I do try to remember to show their BLOOD parents as the primary parents, so they will show up with BLOOD ancestors in their ancestry reports.
As for stepchildren, I recommend to not use the facility within FTM to designate multiple parents followed by using the "step" designation. It leads to unintended consequences, just like you have seen with adoptions.
Note that in the first generation only of a Descendant Register report, you can force a parenthetical phrase for "adopted" for the first generation-child adopted by checking the box for "type of relationship", but that designation will not carry on to further generations - and you will end up with non-blood descendants of an ancestor in the adoptive ancestor's descendant reports and charts.