After the latest changes, I get a Lua error when registering a tutorial, caused by these four lines starting at line 138 in file CustomTutorials-2.1.lua.

frame.TopRightCorner:Hide()
frame.TopLeftCorner:Hide()
frame.PortraitFrame:Hide()
frame.TopBorder:Hide()

Those fields are nil and I couldn't find any reference to them anywhere else in the code.

Classic or retail?

That was on retail, I have not tested on Classic

Ah, those frames might not exist on retail, will test.

Now I'm finding the same visual issue on retail. There's clearly an inconsistency on the blizzard template... goddamit blizz.

Try the new commit and see if the visual bug is gone. I reloaded multiple times and it did not appear.
Will tag a new version if it works.

We're dealing with some high quality blizzard code here... fml