  Validated planet candidates need not lie on long-term stable orbits, and
instability triggered by post-main-sequence stellar evolution can generate
architectures which transport rocky material to white dwarfs, polluting them.
The giant planet HD 131399Ab orbits its parent A star at a projected separation
of about 50-100 au. The host star, HD 131399A, is part of a hierarchical triple
with HD 131399BC being a close binary separated by a few hundred au from the A
star. Here, we determine the fate of this system, and find that (i) stability
along the main sequence is achieved only for a favourable choice of parameters
within the errors, and (ii) even for this choice, in almost every instance the
planet is ejected during the transition between the giant branch and white
dwarf phases of HD 131399A. This result provides an example of both how the
free-floating planet population may be enhanced by similar systems, and how
instability can manifest in the polluted white dwarf progenitor population.
