Sequential Fit should use the same period (or period combination) as selected in the Home tab, plotted in the graph, and used for individual fits.
The result of the sequential fit is the fit to period 1 regardless of the user’s choice.
Load a multiple period file (first of several such runs). Select period 1 to plot.
Fit it to a function.
Do Sequential fit to the set of runs.
Go back to Home and change the period to 2. Re-fit, observe parameters change.
Re-run the sequential fit (with a new label).
Generate the Result tables of the two sequential fits and notice the suspicious similarity.
Mantid 3.9, “Classic” version of the Muon Interface fitting dialog. Seen on Windows, maybe others.