Accelerometer Inference from power spectrum
What could the cause of this trend be? System bias or something else?

Most mechanical systems are low-pass, i.e. they let through less energy at higher frequencies. That would be perfectly plausible, but I don't know what you're observing.
Of course, your accelerometer is inherently frequency-selective, too, and you might need to calibrate it, together with your recording setup.
Lastly, we don't know exactly how you post-process.

Best way of handling this - Matlabs detrend tool does not have much of an effect. What would be the best way to correct this?

&quot;Correcting  this&quot; means &quot;I saw something that is clearly visible in my measurement, but I don't know where it comes from and whether it's actually there in the observed phenomenon. Let me get rid of the evidence&quot;.
That's called a lie.
So, before changing anything about the symptom (plot), make sure you're not masking a cause. This is less about the signal processing than about physically understanding your measurement setup. It can, and probably will, involve you measuring known acceleration amplitudes at different frequencies, and looking at whether the results are as expected, or whether you need multiply with a different factor for different frequencies.