Steps to reproduce

1. Start Minecraft, and connect to a realm (I don't know if it affects single player mode)

2. Switch focus (using Cmd-Tab) to another app, for example Safari

3. Switch focus back to Minecraft

4. Click "Back to game" button

5. Hit "t" button to enter text

6. Type any text containing an "a", e.g. "foo bar"

7. Watch in despair as any text before the "a" vanishes from your input

Is it Reproducible?

Yes, often; but not always. It's not only on my machine; also have confirmed the behaviour on my wife's computer (with Yosemite) and my son's computer (also Yosemite, I believe).

Is it only the "a" character that is affected?

Usually, but at least once I've had the "c" character affected too.

Any screenshot?

Yes, I attached a screenshot of what the text looks like when I've typed "foo ba" – hitting the "a" makes the text change to look like it's become highlighted, and hitting any other character afterwards makes all the text typed so far disappear.