You can test at http://www.tinymce.com and let me know when it's working for you and I will restore it.
I'd say this is a feature to get people to move to a better browser? (erm, Edge isn't /too/ bad however you get my drift).
These links are referral codes
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freitasm: I am told a fix was released. I am on mobile at the moment but if someone can test on TinyMCE.com and let me know please.
I just went there, the letter "Y" is working ok for me.