Inconsistencies in IPA Pronunciation in Text to Speech

Question

Hi,

I'm using SSML to ensure specific pronunciation, however, I'm experiencing some inconsistencies.

For example, here's the word 'would':

<speak version='1.0' xmlns='http://www.w3.org/2001/10/synthesis' xml:lang='en-US'>
      <voice name='en-US-AvaNeural'>
            <phoneme alphabet="ipa" ph="wʊd">would</phoneme>
      </voice>
</speak>

It pronounces the word exactly as expected.

Now if I want to break the word down into individual sounds and just pronounce the 'ʊ' sound, I would use this:

<speak version='1.0' xmlns='http://www.w3.org/2001/10/synthesis' xml:lang='en-US'>
	<voice name='en-US-AvaNeural'>
		<phoneme alphabet="ipa" ph="ʊ">oul</phoneme>
	</voice>            
</speak>

However, now it sounds like it's saying the letter 'O'. I expect that 'ʊ' would be pronounced the same in both cases.

Can anyone offer any insight into why this may be happening? Thank you.