tl;dr: When a tooltip is forced to allocate additional lines, what font object is used for the newly created fontstrings? Public documentation seems completely lacking on this point. Is it as simple as "whatever was used for the (previously) bottommost line"?
So as part of a debugging-mode-only kludge in one of my projects, I redirect the output of the builtin /dump command into a tooltip. The tooltip is of the standard GameTooltip frame type, but it is not the _G.GameTooltip object. Sometimes the tables being dumped get rather large, and this almost always causes more lines of text to be sent than the tooltip previously had allocated.
That's not in itself a problem, because ever since WoW version Old.Ancient.Foreverago, tooltips expand their line count as needed. Where things go wonky is on the really long tooltips, where after a seemingly random (but high) line count, the automatically allocated fontstrings change font.
Before I spend any real time trying to track down what's happening, I'm hoping to learn what's supposed to be happening. What determines the font settings for those on-demand fontstrings?