Most likely it has oxidized to the point of unusability.
Can you share a photo of that part of the label that shows where it was manufactured?
If the bottle was purchased 18 months ago (i.e., mid-late 2023), it would have been sold at the tail end of the Sino Promise era. My understanding, though, is that at that time, Tmax developer (and other Kodak-branded black and white chemistry) was being manufactured in the U.S. by Photo Systems Inc., not China. So in this case, we probably can't blame the anomalous color on poor quality control by Sino Promise.
For what it's worth, I have an open (but capped), 3/4-filled bottle of the same developer that was purchased in February 2024 (roughly six months after the OP purchased his bottle), and the concentrate is still clear and colorless, like new. This purchase was made shortly after PSI (re)released its first tranche of film chemistry on Cinestill's website after signing the licensing agreement with Kodak.
It seems unlikely that PSI would have made a major change to the manufacture of Tmax Developer between mid-2023 and early 2024 that could account for what the OP experienced. I'd be interested to hear how the concentrate was stored (temperature, humidity, sunlight, etc.), as this might help others diagnose or avoid similar problems.