Yeah, it's not a "problem", Nvidia designed the card that way for specific reasons. Like so many technical things, if you want to understand the design, check out Anandtech's write-up:
https://www.anandtech.com/show/8935/geforce-gtx-970-correcting-the-specs-exploring-memory-allocation
The "problem" was that Nvidia initially published incorrect specs for the card and only corrected the issue when enough users had discovered it and generated interest. Many argue it was innocent oversight, some contend that Nvidia deliberately misled the press and public. We'll never know for sure.
In any case, the GTX 970 is still a really good card and remains one of the best selling GPUs of all time. It's just that the last 500MB of VRAM is significantly slower than the first 3.5GB VRAM. Generally that doesn't matter at all, but if you look hard enough you can find edge cases where it makes a pretty big difference. Nevertheless, it's still a solid gaming card.