Software

That is perfectly normal behavior.

"If you name a file "1234;abccd" the file will show as "1234;abcd.docx"."

In Office? Sure, of course, Office will append an Office document extension to the file.

"If you rename the file to say "123.4;abcd" the resulting file is unrelated to Word/Excel yet it can still be opened."

Right. But you are renaming the file Outside of Office now. Of course it can be opened if you choose that application to open it, or open it from within the application, regardless as to what gibberish you put into the extension.

"If you reopen and resave the same file "123.4;abcd", the resulting file name will be "123.4.docx;abcd"

That is sort of odd behavior there, but why on Earth would anyone do that? Semi-colons or not?

Rule #1: Avoid using special characters in a file name. \ / : * ? ? < > | [ ] & $ , .
The characters listed above are frequently used for specific tasks in an electronic environment. For
example, a forward slash is used to identify folder levels in Microsoft products, while Mac operating
systems use the colon. Periods are used in front of file-name extensions to denote file formats such
as .jpg and .doc; using them in a file name could result in lost files or errors.

Rule #2: Use underscores instead of periods or spaces.
As mentioned above, periods already have a specific function in a file name, which is to tell the
computer program where the file-name extension begins. Spaces are frequently translated in a Web
environment to be read as ?%20?. For example,
Naming tutorial.doc
would appear as
Naming%20tutorial.doc
There are a lot more rules, I'd like to add;

Rule #10 Never, ever save any file with the "X" extension from MS ie: .docx,.xlsx,etc.

If anyone else is ever expected to open these files, saved as you described, your user will have a problem.