See the problem?
This is a known issue, with a pathetic workaround suggested by MS; “Log off and then log back on to the current account”. For some unknown reason, tooltip’s seem to lose their WS_EX_TOPMOST style.
Why bugs like this stick around for the lifetime of a product I’ll never understand. Either add the style back in response to the TTN_GETDISPINFO message or fix the underlying problem!
And if anyone is wondering, the Visual Style is Luna Element 4 (Alt. Start button)