[MunkDev]

WorldMapFrame loads before any of your addons, so you'll need to address its overlay frames directly:

local function ShowBountyTooltip(self, bountyIndex)
    print("WorldMapBountyBoardMixin:ShowBountyTooltip " .. tostring(bountyIndex))
end
 
for i, frame in ipairs(WorldMapFrame.overlayFrames) do
    if frame.ShowBountyTooltip then
        hooksecurefunc(frame, "ShowBountyTooltip", ShowBountyTooltip)
    end
end
 
hooksecurefunc(WorldMapBountyBoardMixin, "ShowBountyTooltip", ShowBountyTooltip)