Maximizing tmux session shows weird ANSI Sequences

I have a tmux session inside Putty. It was fine for past many months, but now I see a strange thing. When I maximize a normal Putty window, and if cursor is not on a new line, then I get some weird ANSI Sequences in the shell. Eg, I maximized 4 times to get: 0;44;8m 0;46;8m 0;50;8m 0;55;9m It happens only on maximize, not on restore. If cursor is on a new line, then the codes printed start with "^[[<", & then the ANSI Sequences. Eg, when I maximised 4 times, ensure that the cursor is on a new line, I got: ^[[<0;64;8m ^[[<0;138;8m ^[[<0;95;8m ^[[<0;79;7m What is happening ? I restarted the session and issue is not happening now. How to debug next time it happens ?