AWS Polly Pinyin doesn't say certain things. Is there a document with valid Pinyin?

Take for example 貪小便宜. If I put this text into the console, it correctly speaks it. tan xiao bian yi. However if I input SSML as follows: <speak><prosody rate='slow'><phoneme alphabet="x-amazon-pinyin" ph="tan1 xiao3 pian2 yi2">貪小便宜</phoneme></prosody></speak>, the voice only says tan pian yi, it skips the xiao. I know certain Pinyin can be romanized in different ways, but xiao seems to be pretty standard from what i've seen. I've also tried xiao1-4 none of which worked.

Is there a way to get a list of the valid pinyin for Polly so I can normalize my pinyin to use what is accepted by Polly? I know the voice can say what I'm looking for, as it works when I put the chinese characters in, however for my usecase they won't always be as readily available as the Pinyin, but the Pinyin seems to be less consistent.