Is the word "以后" in "回家以后" an adverb? My question is as titled. Per my understanding the "以后" in "回家以后", "从此以后", etc. is a preposition. So I am surprised the textbook says 以后 is an adverb here. Can anyone find a grammar book, dictionary, or any kind of documents to prove the textbook is correct?
I can't find a resource to cite from, but here's what I know.
In some grammar textbooks, whether Chinese or English, words like this are referred to as a Prepositional Adverb, or words that are very similar in its form to a preposition but functions as an adverb.

EDIT
I am not certain about this, but some say that 以後（后） is one of the post-positions, which corresponds to prepositions in English.

Consider 以後/after in the following sentences:

以後(after) as a preposition.


好了以後，記得打給我。
Call me after(when) you're done.


&quot;After&quot; as an adverb.


I will arrive soon after.
我馬上到。


以後 as an adverb. (?)


以後，我會再打給你。
In the future, I will call you again.