Yes, it is perfectly possible for an attorney acting under LPA/POA to "tweak" the assets so that they potentially improve their own lot as a beneficiary at the expense of another, should they know the contents of the will. But that is an extreme case, and requires a dishonest attorney, which the vast amount aren't, despite what we occasionally read here on the forum!
It is also possible for a Deed of Variation to be made, with the agreement of the potentially disadvantaged beneficiaries, to make adjustments to the distribution of a will in order to put a situation such as being left out of a will (in this instance, where the asset no longer exists) right:
https://www.gov.uk/alter-a-will-after-a-death
However, if the Rembrandt has been sold, or the diamond ring has disappeared, or the ISA account closed, it really does not exist any longer as a bequest - unless in extremis it can be proven that the act was carried out deliberately to disinherit a beneficiary, using our example of a dishonest attorney.