Muntjac are native to deciduous woodland. They are principally browsers and need a decent understory to browse on. The are not grazers like red or sika.
Conifer plantations really offer very little in the way of food for browsing animals. Conifer is not very good palatable. Really the only food is what grows in the rides and odd bits of clearings - fine for red and sika, but hardly surprising that browsers such as muntjac wouldn’t survive.
Deciduous woodland along river valleys and indeed roads and railways another thing entirely.