To reproduce:

Public Class Form1
    Sub New()
         
        InitializeComponent()

        Me.RadScheduler1.EnableExactTimeRendering = True
        
        Dim dt = DateTime.Now
        Me.RadScheduler1.Appointments.Add(New Appointment(dt.AddMinutes(-10).AddSeconds(1), Now, "A1"))
        Me.RadScheduler1.Appointments.Add(New Appointment(dt.AddMinutes(-15), DateTime.Now.AddMinutes(-10), "A2"))
        Me.RadScheduler1.GetTimelineView().StartDate = DateAdd(DateInterval.Minute, -15, Now)

        Dim customScale As CustomTimescalePerMinute = New CustomTimescalePerMinute()
        Dim timelineView As SchedulerTimelineView = Me.RadScheduler1.GetTimelineView()
        timelineView.SchedulerTimescales.Add(customScale)

        timelineView.ShowTimescale(customScale)
        Dim currentScaling As SchedulerTimescale = timelineView.GetScaling()
        currentScaling.DisplayedCellsCount = 15
    End Sub
End Class

Public Class CustomTimescalePerMinute
Inherits MinutesTimescale
    Public Overrides ReadOnly Property ScalingFactor() As Integer
        Get
            Return 1
        End Get
    End Property
End Class

Note: the attached gif file illustrates the behavior. Initially, the first appointment's end is not rendered in the correct time slot. After scrolling, it is displayed correctly.

Workaround: force scrolling

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    Dim t As SchedulerTimelineViewElement = TryCast(Me.RadScheduler1.SchedulerElement.ViewElement, SchedulerTimelineViewElement)
    Dim value = t.NavigationElement.Value
    t.NavigationElement.Value += 1
    Me.RadScheduler1.SchedulerElement.RefreshViewElement()
    t.NavigationElement.Value = value
End Sub