Why do 20-series Learjets have such an extreme susceptibility to mach tuck?
The early series 20 Lears have always had boundary layer separation issues, so much so that any time the wings are removed and reinstalled the aircraft must be test flown. Mach tuck is an issue with all aircraft capable of trans-sonic flight, but on the early Lears this is apparently exacerbated by the susceptibility to boundary layer separation from the laminar flow airfoil.

To address this and other shortcomings in the 20-series wings, a number of modifications have been developed. Vortex generators were installed in front of the ailerons, and then a boundary layer energizer "fence" was developed. Dee Howard and Gates Learjet jointly developed a "cuff" for the Lear 25 wing that resulted in the 25G variant, with better performance at cruise and an increase in range of some 360 nautical miles. 

There has been some talk that the problems of the Lear wings come from the FFA P16 design. Bill Lear's son has clarified that his father did not use the P16 as the basis of the Lear 23. The two aircraft use different, though similar airfoils.