"It is therefore typical of he/she to ....,being (serious, ect) all this while"
"Being a (characteristics,e.g. serious) person, it is typical of he/she to..."
"As it is known for his/her 〜, he/she will therefore .... "
"Known for his/her 〜, he/ she will therefore .... "
This is the only acceptable use of this grammar: Ｎ + の + ことだから
「Ａ(の)ことだから、Ｂ」 The case where Ａ is a person occurs often. Ｂ predicts or anticipates the nature or usual circumstances of Ａ.
誠 is correct. This grammar only means "as always" when の is used in front of it. The other uses of ことだから, such as the examples that 誠 pointed out, have a different meaning.
In modern day Japanese, you can say ことだから without using 事.
Only ex#251 fits into this category of grammar, ex #3788, ex #3790 , ex #3792 all not examples of this grammar pattern, they are こと(事） ＋ だから。Therefore is : ex#3788 : そういうこと ＋ だから ex#3790 : 大事なこと ＋ だから ex#3792 : やったこと ＋ だから The above 3 examples, こと can be replaced by 事