FTL's (and goodness knows where this theory was hatched?) are on a rolling daily weekly annual or 28 day cycle. They do not ordinarily reset at any point.
Duty periods are quite distinct from Flying duty periods and it is normally the latter which is the restrictive element although the former is used for subsequent rest calculations.
In an attempt to steer this subject back on course.... Why did the authors operator, acknowledge request of a refund? Was it a case that the outbound flight had been cancelled and a refund had been requested for the booking reference with the assumption that this was a round trip cancellation? Could it have been a generic letter used when it should have covered a more specific situation? Did it just come out of the blue without any other action having precipitated it from the author?
There must be more to it, and I am sure it has nothing to do with Flight time limitations?