  We study the time-variable linear polarisation of Sgr A* during a bright NIR
flare observed with the GRAVITY instrument on July 28, 2018. Motivated by the
time evolution of both the observed astrometric and polarimetric signatures, we
interpret the data in terms of the polarised emission of a compact region
('hotspot') orbiting a black hole in a fixed, background magnetic field
geometry. We calculated a grid of general relativistic ray-tracing models,
created mock observations by simulating the instrumental response, and compared
predicted polarimetric quantities directly to the measurements. We take into
account an improved instrument calibration that now includes the instrument's
response as a function of time, and we explore a variety of idealised magnetic
field configurations. We find that the linear polarisation angle rotates during
the flare, which is consistent with previous results. The hotspot model can
explain the observed evolution of the linear polarisation. In order to match
the astrometric period of this flare, the near horizon magnetic field is
required to have a significant poloidal component, which is associated with
strong and dynamically important fields. The observed linear polarisation
fraction of $\simeq 30\%$ is smaller than the one predicted by our model
($\simeq 50\%$). The emission is likely beam depolarised, indicating that the
flaring emission region resolves the magnetic field structure close to the
black hole.
