9,200 sq meters of land in Tax Declaration & Deed of Sale vs 8,836 sq.meters in Land Title

Hi, my mom bought a land and the size is 9,200 sq.meters as stated in Deed of Sale & Tax Declaration. When she had the land surveyed by the DENR people and the title came out, it became 8,836. My mom was upset about it as I can recall. Now my mom passed away and one of the reason is this fencing dispute that made our property smaller. I had the place re surveyed by a private geodetic engr. based on the fence created by the neighbor (whose sister is a DENR employee and a fixer and who is also the one who assisted my mom in processing the papers and surveying before, because she was a relative!)
The result was exactly 8,836. But my mom pointed out to her sister before, the original "muhon" and the neighbors new the original "muhon". When we had the original muhon surveyed, it did match the 9,200 sq. meters.
If there is a document signed by my mom about possible right of way bec her land is right smacked in the middle of a huge subdivided land, I don't see any document, and I have requested for all documents pertaining to this property in Region 10 office, but there is none.
So how did the neighbor get our 336 sq meters?

What are the possible cases I will file against the DENR employees who surveyed, and processed the land title?

How can I get back the missing 336 sq. meters?

I was thinking of connivance for the DENR relative bec she made the size reduced in favor of her siblings' property.

I also informed the neighbor that it was against the law in putting up the fence without informing the rightful owner of the neighboring lot.

Thanks