a periodic anomaly in coherence plot changes with changing the window size

When I plot coherence between two signals, I get the result as 1 for all the points, which is acceptable, but the problem is that the plot includes a periodic anomaly. When changing the window size, the period of that anomaly changes. i.e. when the window size is 512, i get the coherence value less than normal, which is 1, and that anomaly value repeats every 7 points(picture 1). When the window size is 256, that anomaly repeats every 3 points(picture 2). What is the reason for that anomaly and how to get rid of it?