I thought that ㅅ is pronounced /t̚/ at the end of a syllable block, but it seems to be an /s/ in 것은, even though 것 on its own is /kʌ̹t̚/.
Does it depend on word boundaries, or is it just irregular?
ㅆ (/s͈/) and ㅅ (/sh/) are pronounced as /t̚/ whenever the following conditions are all met:
This means that:
Refer back to your pronunciation rules for this one. You're correct that ㅅ and ㅆ exhibit closure as 받침. But in this case, you have a ㅅ and an ㅇ next to each other in the order that they will be read. For that reason, re-syllabification occurs.
So, instead of 것은 -> 거-뜬 or 거-튼, it's 것은 -> 거슨.
Hope this helps!