Why does fzf output wrong lines in tmux?

Question

When I do the following...

1. Create a list of random words (randomwordgenerator.com)
2. Copy it into my Windows clipboard
3. Start two instances of my Cygwin minTTY
4. Start tmux in one of them.
5. Run the command cat /dev/clipboard | fzf in both terminals
6. Type some letters to filter the fzf list

I get the expected result in the non-tmux terminal:

But in the tmux terminal, there are display glitches. All words above the blank lines are not supposed to be there, and they are not selectable as well:

Moreover, when I move up in the tmux terminal with the arrow keys, winnir (wrong) gets replaced by significance like in the other window.

Why is this happening? Is there anything I can do about it? Or is it a feature I simply do not understand?