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This looks like a internationalization bug in Outlook.
Outlook is programmed to remove any prefix containing 3 or less alpha characters, followed by a colon (RE:, FWD:, NET: etc). So, under normal conditions, Outlook will strip any prefix.
In your case, apparently the translator to Vietnamese made the mistake of
translating RE
as Trả lời
, a mistake that breaks this convention.
You may report this as a bug to Microsoft. There isn't much more that you can do.
It’s best to stick with RE and FW as a courtesy to other people. Outlook lets non-English language users do that with two settings found in File > Options > Advanced > International options: