Brad Smith this should help:
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07.html#:~:text=Vietnam%20protects%20citizens%20affected%20by%20online%20scam,in%20Vietnam:%209%2C700%20reports%20in%20two%20weeks.
It is entirely rational given all the circumstances. As a Viet passport holder she is entitled to the 60 days (and was stamped as such), but there is increased sensitivity because of the crackdown on scams and human trafficking.
The reduction to 30 days reflects the possible/probable reduction. From immigrations PoV they are screening for scammers and when they are content, they reduce the risk further by imposing conditions of entry (well within their power). Many Viet passport holders have been detailed or turned around.
Hope that helps!