When you put the unicode symbol ⇄ inside a button, it turns the button square <button>⇄</button> Chrome, OSX.
Please attach an HTML test case. Does this reproduce in Safari?
Created attachment 289284 [details] simple test case
Just tested, and yes this occurs under Safari as well.
I can't reproduce in Safari on macOS Sierra.
I'm on OSX 10.9.5
This doesn't reproduce with Safari 9.1.3 on OS X 10.10.5 for me. I don't have OS X 10.9 to easily try there.
Created attachment 289295 [details] screenshot
That's a really old Safari.
I am not able to reproduce this bug in Safari 16.3, Safari Technology Preview 164 as compared to reference screenshot using attached testcase 'simple test case'.