Clemmensen reduction of α-, β-keto acids
The likely reason for this is a steric hindrance. The Clemmensen reduction is very vulnerable to hindrance. See the image below: 
There was also another reaction conducted that had a 67% yield for a gamma keto acid with resonance. See the image below:

Therefore, the main factor that makes α- or β-keto acids not undergoing Clemmensen reduction is likely not because of resonance, but primarily of steric hinderance. But this also may raise another question, why doesn't the carboxylic acid group get reduced? The reason for this is that the carbon in a carboxylic acid is not quite partially positive because OH is an electron-donating group. The mechanism involves a transfer of an electron from zinc to the partially positive carbon atom, which, as mentioned, carboxylic acid doesn't have.
However, even for diketones, the Clemmensen reduction doesn't often yield what one would expect. See the image below:

The reason is because of the mechanism. In the mechanism, radicals are formed on the carbonyl carbon. However, if you have a diradical, they can come together and form a bond if they are close enough. This is just some extra information.
The source I took the images from: https://www.sciencedirect.com/topics/chemistry/clemmensen-reduction#
Extra papers I found that may be helpful:
https://d.lib.msu.edu/etd/37284/datastream/OBJ/view
https://pubs.acs.org/doi/abs/10.1021/jo01017a512