Model wrongly declared as "not watertight"? (Tinkercad model, with text shape generator)

[Cyberman-tM]

A few models gave the "not watertertight" warning. After some digging, I'm convinced the troublemaker is the "text" shape generator from Tinkercad - apparently it creates some wrong vertices. (Unfortunately not an issue they want to fix.)

Mesh tools almost always declares the model to be not watertight. I've tried to double-check, but no other tool had the same issue. Most of them agreed that there is something wrong, however, so maybe it depends on how the program decides to continue after the error?

Here's a simple model that shows the issue:
watertight_or_not.zip
(zipped because STL is not allowed)

If you want to see the original, here's the link to Tinkercad:
https://www.tinkercad.com/things/eRpr98AxZoG

[fieldOfView]

3D Builder agrees with MeshTools that there is something "fishy":

I'll try to find out what is "wrong" with the model created by tinkercad.