Local acyclicity in $p$-adic cohomology

We prove an analogue for $p$-adic coefficients of the Deligne–Laumon theorem on local acyclicity for curves. That is, for an overconvergent $F$-isocrystal $E$ on a relative curve $f:U\to S$ admitting a good compactification, we show that the cohomology sheaves of $\mathbf{R}{f}_{!}E$ are overconvergent isocrystals if and only if $E$ has constant Swan conductor at infinity.