Indeed, this is a bug in the Lightweight RenderMode of the tooltip manager and I am logging it for fixing: FIX Loading sign is not visible when height or explicit scrolling is set in a RadToolTipManager with RenderMode=Lightweight. I hope the workaround I have provided will be suitable for you:
I have also updated your Telerik points for your report.
Of course, you can, alternatively, fall back to the Classic RenderMode.
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