Ideas
I have been experiencing the exact same situation that you have described on an RTX 5080. (16GB VRAM) I am running the game on 4K with DLSS-4 Quality enabled. All of the graphics settings are set to overkill along with the texture quality setting. When i launch the game initially, the VRAM slider found in the graphics settings tab indicates a VRAM usage of approximately 9GBs. As i continue playing the conquest mode, regardless of the map, this value gradually increases and indicates that the game occupies 15.4 GB of VRAM. According to my observations so far, the longer i play, the more drastic this occupance becomes, which eventually causes hitches, stutters and unusually low fps counts (Drops from around 100-110 fps to 70s in my case.)
This problem appears to mostly affect PCs with high end specs, particularly users who run the game in Ultra/Overkill graphics presets on high resolutions.
This condition perfectly fits the definition of a "VRAM Memory Leak" as you described.
My system specs are as follows: Intel Core Ultra 9 285K, ASUS Prime RTX 5080 (with 16 GB VRAM), 48 GB GSKILL CL32 6400 MHZ RAM and 2TB Samsung 9100 PRO.
This issue still persists to this day and the community would be grateful if the developers addressed this issue.