Block state values don't autocomplete unless you type a space after the colon

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In Java Edition, you are also required to input a space after using an autocomplete suggestion in order for more suggestions to be displayed. Why do you believe this is a bug in Bedrock Edition?

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Suggestions appear immediately after typing the equals sign

In Java Edition, after pressing Tab to autocomplete "axis=", the x/y/z suggestions do not appear until a space is added.

That's true. It's because tab cycles through suggestions, it doesn't switch to a different set of suggestions. You'll note that if you type backspace after the space, or type the state name manually, you still get suggestions. This doesn't happen in Bedrock.