svn merge branch A -> B erases changes in B, what is wrong?

I needed to merge updates from master branch A to feature branch B. I did it by VCS -> Integrate Project command
Source 1 was branch B url, Source 2 was branch A. After command completed I saw that all new changes in feature branch B were erased
I wonder why it happend and what's the right way to merger from master to feature branch in Idea?

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