I have noticed this same feature.
I have only seen it in OSX. In fact, I have only noticed it in OSX 10.3.x.
Not sure if this works for all applications or just ArchiCAD. I was frustrated at first by it as the names of my files kept changing on me somehow. Now that I understand and know when it happens I am better able to navigate around my directories without changing my file name by accident.
Can be a very neat feature or extreme nuisance.
Terrence Sturm, Architect
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MBP OSX 10.15.4 Quad Core Intel i7 2.2hz
AC 17 build 5019
AC 22 build 7000
AC 23 build
AC 24 build 5000