Try opening up an active probe and looking at the soldering, these low volume boards with low numbers of components as used in the matching networks (termination) are usually hand assembled and look just like the pictures posted so far. Unwashed flux? thats the recommended application for many fluxes!
Not to push one brand over another here, but I opened up my Lecroy 2.5GHz active probe a while back and it was pristine on the inside. I'm sure any of the $3000 keysight probes would be the same.
Last week I opened one of the Keysight N2843A 500MHz probes that came new with my 3000T, and it was pretty messy. Not quite as bad as sergio's pics, but close. Pretty disappointing. Clearly it was made CHEAP.
But hey, it works and is up to spec so what can I really complain about?