The 'index.html' filename and a bug I encountered with it (or not?)

Hello there!
I’m currently working on the “Tea Cozy” self directed project module for part 4 of the web development path and I noticed something odd.
I was trying my best to both type out my own code and using the pre-made html and css within the project and I found it weird that in the beginning, nothing was displaying correctly or as expected.
When I copy pasted the pre-existing code within the assignment, it all showed up just fine. I copied the code onto a separate file and renamed it ‘INDEXTWO.html’ in comparison to my original ‘index.html’. Same code, different filename, and for some reason the html that was under ‘index’ did not display correctly. When I renamed that file into ‘indexy’ (or any other name really) only THEN it actually displayed correctly.
I just thought it was something interesting to talk about, and is there a reason why this happened in the first place? I never had this issue with past projects.

Thanks for reading!

Sounds like there are some linking issues in your original code. Please provide your html code and folder tree.