[folia] word.previous() violates constraint when given an explicit constraint like Sentence

[proycon]

I’ve encountered some problem with the usage of previous() in Folia 
As far as I see if the token is the first word of the sentence, and doesn’t return None, instead, it returns the last word of the previous sentence and sometimes even a word before that
and by the way, I explicitly define the sentence as a constraint like this : token.previous(folia.Word, [folia.Sentence])
This works well for the next, it returns None for the next token if the current token is the last word of a sentence. So “token.next(folia.Word, [folia.Sentence])“ works well
here are my observations:
flooding . None
 --------------------
 Prev | Token | Next
 --------------------
 flooding There was
here ‘flooding’ is the last word in the first sentence, and ‘There’ is the first word in the second sentence)

and for your information, previous() sometimes returns even before than the last word of the previous sentence
Here is the exapmle I’ve seen;
two days of
days of intense
of intense rainfall
intense rainfall .
rainfall . None
--------------------
Prev | Token | Next
--------------------
days A very
A very slow-moving
very slow-moving lo