I miss having a button to compose. Menu/compose, eh.
Do you have a very small screen? I do get a button to compose directly in the top toolbar (when looking at the message list)
If you increase Android’s “display size” setting, there’s less room for UI elements (because they’re displayed larger). So from the apps perspective you do have a small screen. The action bar component contains some logic that decides how many actions to display outside of the overflow menu (three dots). It depends on the available space. For your (virtual) screen size the answer is: one.