Don’t know Greek, so need to ask if it an entirely different term or as different as happy is to happily? Just wondering, and if it is used in any other historic texts for context?
Nouns in Koine Greek derive from the verbs. The first few bibles had the definite article ‘the’ in front of the ‘apostasi’. so we know immediately this refers to a specific thing or event. When the word is changed along with the article to an indefinite article “a” the meaning is blurred out from the original intent. The use in Acts 21:21 refers to a process whereas in 2Thess2:3 it refers to a specific thing or event.