To Reproduce

1. Start typing.
2. Enable italic via CTRL + I or clicking the italic button
3. Type a bit in italic
4. Attempt to enable bold with CTRL + B or clicking the button
5. Continue typing.

The exact buggy behavior seems to differ between browsers.

In Chrome, the current style (italic) is closed and the new one (bold) is started so you get italic text followed by bold text but not bold italic text.

In Firefox, the current style (italic) simply continues, ignoring the new one (bold), so you get only italic text. (Pictured below)

Notes:

1. Attempting to record a GIF of this behavior with LiceCAP sometimes made it work, but not always (see below).
2. Steps 2 and 4 above can be reversed and show the same issue.

Expected behavior
It is already possible to nest bold and italic if all the words are written and then formatted via text selection. I should be able to produce the same result as I write.

Screenshots
Firefox:

Desktop (please complete the following information):
Win 10, Firefox 63 and Chrome 70 (behavior varies, see above), WP 5.0-beta3, Twenty Nineteen, no plugins.