Seems to me the behavior of the "Select all" shortcut Cmd/Ctrl + A is very buggy across different browsers and platforms, at a point that I doubt it can be considered ready to be released in its current state.

Describing what happens is a bit difficult because the actual behavior greatly varies across browsers and platforms, sometimes also with the same browser on different platforms. To my understanding, a first cause could be the way a keydown event works on modifier keys on different platforms. For example, in my testing on macOS when pressing and holding Cmd, the keydown occurs just once. Instead, on Windows when pressing and holding Ctrl, it occurs continuously.

Regardless, here's this is what is the test case I've used and the expected behavior:

• click on a block, for example a paragraph
• press Cmd (or Ctrl on Windows) + A: the block content is selected
• click on another paragraph
• press Cmd (or Ctrl on Windows) + A: the block content is selected
• press again Cmd (or Ctrl on Windows) + A: all the blocks get selected
• note that the last point should work either if Cmd (or Ctrl) has been hold or released and then pressed again

Seems to me this behavior works correctly only with Chrome 67 on macOS.

Firefox 60.0.2 on macOS:

• click on a block
• press Cmd + A: the block content is selected
• click on another paragraph
• press Cmd + A: the block content is selected
• press again Cmd + A: at this point looks like the block is not "selected": the UI disappears. Expected: all blocks are selected
• press Cmd + A for the third time: finally all the blocks get selected

Safari 11.1.1 (on macOS of course):

• click on a block
• press Cmd + A: the block content is selected
• click on another paragraph
• press Cmd + A: the block content is selected
• press again Cmd + A: nothing happens: the block UI stays visible and only its content is selected. Expected: all blocks should be selected

Chrome 67 on Windows 10:

• click on a block
• press Ctrl + A: the block content is selected
• click on another paragraph
• press Ctrl + A: all blocks get selected! Expected: only the block content is selected

Firefox 60.0.2 on Windows 10:

• click on a block
• press Ctrl + A: the block content is selected
• click on another paragraph
• press Ctrl + A: the block content is selected
• press again Ctrl + A: at this point looks like the block is not "selected": the UI disappears
• press Ctrl + A for the third time: finally all the blocks get selected
• until now the behavior is the same of Firefox on macOS, but...
• with all the blocks selected, click outside of them to cancel the selection
• click on a paragraph
• press press Ctrl + A: all the blocks get selected! Expected: only the block content is selected

IE 11 (similar to Chrome on Windows):

• click on a block
• press Ctrl + A: the block content is selected
• click on another paragraph
• press Ctrl + A: all blocks get selected! Expected: only the block content is selected

Sorry I can't test with Edge at the moment, because of a recurring issue with VVV.

Considering the important impact a so buggy behavior may have on writers used to use the keyboard and users who prefer or have to use the keyboard, I'd tend to think this feature should be either fixed ore removed from the final release.