  A highly sheared dense aqueous suspension of granular cornstarch particles
displays rich nonlinear rheology. We had previously demonstrated the growth and
onset of interfacial instabilities when shear-thinning cornstarch suspensions
were displaced by a Newtonian fluid, and had suggested methods to maximise
displacement efficiency [Palak, R. Sathayanath, S. K. Kalpathy and R.
Bandyopadhyay, Colloids Surf. A Physicochem. Eng. Asp., 629 (2021) 127405]. In
the present work, we explore the miscible displacement of a dense aqueous
cornstarch suspension in its discontinuous shear-thickening regime in a
quasi-two-dimensional radial Hele-Shaw cell. We systematically study the growth
kinetics of the inner interface between water and the cornstarch suspension,
and also of the outer interface between the suspension and air. In addition to
the growth of interfacial instabilities at the inner interface, we observe a
transient withdrawal of the suspension and the formation of fingering
instabilities at the outer interface. We demonstrate that these `reverse
fingering' instabilities are extremely sensitive to the injection flow rate of
water, the gap of the Hele-Shaw cell and the concentration of the displaced
cornstarch suspension, {and emerge irrespective of immiscibility between the
fluid pair. We believe that as the cornstarch suspension dilates due to the
high shear rate imposed by the displacing fluid, the outer suspension-air
interface responds with a restoring force, resulting in the penetration of air
into the suspension and the formation of reverse fingers. We note that the
growth of reverse fingers significantly reduces the displacement efficiency of
the suspension. Finally, we demonstrate a correlation in the growth of inner
and outer interfacial patterns by computing the velocity with which stresses
propagate in the confined dense suspension. Our findings are useful in
understanding the flow of granular materials through constrained geometries and
can be extended to study stress propagation in shear-thickening materials due
to a sudden imposition of high shear rate, such as in impact behaviour.
