Hello Bunny,
What a surprise. I also happen to have mm80 bought in September 2023.
I would say, that mine is very loud.
I have changed the original strings to Worth Browns with Low G (BM-LG 46). But the originals were also loud.
Overall this is my loudest concert, and I assign it to the much bigger body than all my other concert uks.
The ukelele looks great, but mine plays all the notes sharp (except on the open strings).
It turned out, that I had to extend the saddle by several millimeters further from the soundhole to fix that issue, because the chords sounded terrible.
I suspect it must be some kind of a building mistake.
In your case, maybe it is the strings, or maybe yours also has some flaw.
Personally I will never trust in high quality of anything from China again.