@mariosv, thank you (again) for taking the time to help me!
First of all, I tested the OpenGL status in LibreOffice 5.3.5.2 x64 (that I had reverted to), and confirmed that it did not cause the problem, no matter how it was set.
Next, I uninstalled version 5.3.5.2 x64, redownloaded version 5.4.0.3, and installed the latter. Now, I cannot replicate the problem in version 5.4.0.3! Everything works fine, no matter what the OpenGL status is (it is off by default).
I am guessing that what may have originally caused the problem, was multiple instances of LibreOffice 5.4 running (I think I recall seeing them in Task Manager), but cannot be sure. Neither do I have any idea as to what caused the multiple instances of LibreOffice to be present, I do not recall anything crashing while I installed version 5.4 the first time.
Now, I am rather ashamed that I did not carry out more extensive research before posting. In any case, thank you very much, I confirm that editing a cell works just fine in Calc 5.4.0.3, with no visual artifacts whatsoever!