When someone leaves an employer while under bond, they trigger a liability. If your new employer offers to remove some or all of that liability to the old employer and replace it with a new potential liability in the form of a bonding agreement with them, I do not see how there can be a benefit in kind which would generate a tax liability.
I have never worked for a company rich enough to clear new employees' bonds with previous employers, but I do know of large UK AOC holders, both fixed and rotary wing, who do this for both pilots and engineers. I cannot imagine that they do so without being confident that HMRC are not going to pursue them for tax on a benefit in kind (it being easier for HMRC to pursue the employer rather than the employee).
RHRP