Hi, a user of my little addon "persistent-world-map" today reported that there seems to be some incompatibility.
https://www.curseforge.com/wow/addons/persistent-world-map#c12

Can you find out what is causing this on your side?

OK! Thanks. I guess
EncounterJournal:SetPoint("TOPLEFT", UIParent, "TOPLEFT", 16, -116)
achieves the same effect for me as
ShowUIPanel(EncounterJournal)
HideUIPanel(EncounterJournal)

On top, you could make your code robust against premature calls of ShowUIPanel(EncounterJournal) by checking for the existence of self.instancesLockouts.

This happens because of your ShowUIPanel(EncounterJournal) call on login. I am hooking its OnShow on ADDON_LOADED and therefore the instance data isn't yet available at this point (PLAYER_ENTERING_WORLD is triggered before UPDATE_INSTANCE_INFO).

function AddOn:UpdateSavedInstances()
  self.instancesLockouts = {}

  [...]
end

function AddOn:UpdateInstanceStatusFrame(instanceButton)
  [...]

  local instances = self.instancesLockouts[instanceButton.instanceID] -- attempt to index field 'instancesLockouts' (a nil value)
  if not instances then return end

  [...]
end

local function UpdateFrames()
  local b1 = _G.EncounterJournalInstanceSelectScrollFrameScrollChildInstanceButton1
  if b1 then
    AddOn:UpdateInstanceStatusFrame(b1)
  end
  for i = 1, 100 do
    local b = _G["EncounterJournalInstanceSelectScrollFrameinstance" .. i]
    if b then
      AddOn:UpdateInstanceStatusFrame(b)
    end
  end
end

frame:SetScript("OnEvent", function(_, event, arg1)

  [...]

  elseif event == "ADDON_LOADED" and arg1 == "Blizzard_EncounterJournal" then
    _G.EncounterJournal:HookScript("OnShow", UpdateFrames) -- conflict happens here
    hooksecurefunc("EncounterJournal_ListInstances", UpdateFrames)
  elseif event == "BOSS_KILL" then
    RequestRaidInfo()
  elseif event == "UPDATE_INSTANCE_INFO" then
    AddOn:RequestWarfrontInfo()
    AddOn:UpdateSavedInstances()
    UpdateFrames()
  end
end)

And this is what's conflicting:

startupFrame:RegisterEvent("PLAYER_ENTERING_WORLD")
startupFrame:SetScript("OnEvent", function()

  -- Got to call this once to bring the frames into the right position.
  -- Otherwise it will be misplaced when Show() is the first called function.
  ShowUIPanel(EncounterJournal)
  HideUIPanel(EncounterJournal)

This is also a pretty hacky way to solve your issue.

I think showing a frame on PLAYER_ENTERING_WORLD is a very uncommon practice and shouldn't happen at all because the player can't even see it at that moment. So I don't think I should do a nil check here.

I also think SetPoint is the right way to handle your issue, thanks for changing it. Our add-ons should not conflict now.

Cool. Which event would you use to show a frame right after the client finished loading? Is there anything after PLAYER_ENTERING_WORLD?

Not sure really, I never thought about it. There are a lot of events after PLAYER_ENTERING_WORLD but the point is, you shouldn't show a frame in that time lapse anyway, for whatever reason. If the user cannot see it, there is no point in displaying a frame.