Bug Unity 2018.4.16 - severe framerate issues on RTX cards

Hello,

we have run into a performance-related issue on RTX cards in our game. Namely, the framerate plummets from 140 to 15 after around 30 to 40 minutes of playtime. The reproduction of this issue involves loading and unloading multiple scenes in-game in the aforementioned timespan. The problem could also potentially influence other software - for instance, the moment the issue becomes present, a video being played on another display on YouTube using Google Chrome will start to drop its framerate whenever a scene starts loading in the game. Additionally, with the issue being present, the loading time for the scene increases from 1-2 seconds to 3-4 seconds. We have tested it using a GPU profiling software, and according to it the GPU idles during the framerate issue.

The issue persists only on RTX series graphics cards, as we have tested it on other hardware. We are currently running Unity 2018.4.16, and have undergone multiple migrations, both to 2018.4.24 and Unity 2018.4.36, which have not resolved the issue. What is interesting however, is that we have tried a migration to Unity 2021.3.3f1 where the issue was not present anymore, so it seems to be related solely to Unity 2018.
While it fixed our issue, it brought many other problems that would take a lot of time to fix, so I would like to ask if this is a known problem, and if yes, is there a particular solution that does not involve upgrading the engine version?

Thanks!