the first sign of something ending
"Ending" is a gerund-participial clause modifying "something". Semantically, such clauses are similar to relative clauses: compare the first sign of something that is ending.
But we don't call them relative clauses since there's no possibility of them containing a relative phrase (cf. *"the first sign of something that ...
The more formal version is the following:
1. The point is . . .
But at some point, this got shortened:
2. The Point is being . . .
→ Point being . . .
The short form now has informal and idiomatic usage.
Most likely, the confusion over adding is back in again occurs when people get stuck in an intermediate state between the original version and ...
I think this is a case where Englsh is in flux.
Historically, this is an absolute clause, with being as the (non-finite) head of the clause: a finite verb ('is') is not needed, and in fact not grammatical. This construction is still used in rather formal writing.
But many English speakers are not familiar with this construction, and (I think) reanalyse ...
The first sign of something ending comes from:
The first sign of something [that is] ending.
It is a reduced relative clause.
reduced relative clause
And here is one rule for them from the "Grammar Bank" site:
If "To be" verb is used after a relative pronoun we can omit "Relative Pronoun + To be".
The car which is parked next to mine is very expensive....
"Having been a teacher, she likes children" seems to indicate that her liking of children is dependent upon her career (now ended) as a teacher. As if should she had chosen another career, she would not like children.
"Having been a librarian, she dislikes children" Presumably had this person NOT become a librarian she might like children.
The first ...