Why is entire Investment Bond being encashed / liquidated?

There's a high likelihood that the encashment/liquidation you're seeing is an unintended consequence, or side effect, of the software's withdrawal policy (for maximising tax-efficiency), as follows:

By default, when one enters an Onshore, or Offshore Bond, in the first instance, the software assumes that the policy contains only a single 'segment' (see Advanced Settings > Life Fund Segments). If, at some point during the plan, the software determines that it's necessary to take an ad hoc withdrawal (a withdrawal that exceeds the client's accumulated tax deferred allowance), it will then seek to determine the most tax-efficient strategy for the withdrawal. If it determines that 'liquidation of complete segments' is the most 'tax-efficient' strategy, this will have the 'side effect' of en-cashing the entire bond.

The unintended liquidation is easily resolved: simply select the relevant Bond, in the Ledger, within the Investments screen. Then, under Advanced Settings > Life Fund Segments, either enter the correct number of segments, or (if you're not absolutely certain) make a plausible assumption with regard to the number of segments within the policy.

If the segments have been entered, and the Bond is still being en-cashed / liquidated, it may be that the Bond has been inputted incorrectly. Note that, in the small Time panel, one must select either one, or more, 'Stages', or two 'Events' (i.e. both a 'Start', and an 'End' event). Where a Bond is jointly-owned, for example, the Time panel might look as follows: