Popover Arrow not showing for positions left and right but shows only for top and bottom

Question

I have a simple popover button but the arrows don't show when displacement is set to either left of right.

<button type="button" class="btn btn-default btn-secondary"
            [popover]="'Vivamus sagittis lacus vel augue laoreet rutrum faucibus.'"
            popoverTitle="Popover on right"
            placement="right">
      Popover on right
</button>

import {PopoverModule} from "ngx-bootstrap/popover";
@NgModule({
imports: [PopoverModule]
})

expected result

actual result