Are bridge bonds in diborane longer than the terminal H bonds? The compound diborane on alkylation loses the terminal $\ce{H}$-bonds rather than the bridge bonds indicating that the bridge bond is stronger but on the other hand it's length is found to be longer than terminal hydrogen bonds. But usually bond length is inversely proportional to bond strength.

What is the reason for this?
As ocresol said, its not wise to conclude the bond strength from one reaction. Rather, considering the structure of the molecule is a more viable option.
Actually the bridged bond is weaker than the terminal $\ce{B-H}$ bonds. This is because of the presence of multielectron density region. Since it is a pair of shared electrons spread between two regions, so more volume and less the electron density. 
  

Image Source: Quora.
Also, the bond order of terminal $\ce{B-H}$ bonds is 1, whereas the bond order of the bridged bond is half of it ie. 0.5. And it is intuitive that bond strength is directly related to bond order.