I have this same issue. I have an RX Vega and it's reading 17GBs on some games that I play as well, like FFXV for example. I'm not sure what it's getting the number from but it doesn't seem to be accurate from the beginning and as you continue to play over time, that number just contiues to climb and stays high even in other games. I know it's not accurate because Task Manger, HWInfo and even the FPS overlay from FFXV it self are all reading the same VRAM usage except AMD Overlay
The VRAM reading is not reliable in this overlay, needs a fix I think.