Steps to reproduce:
1. Open attached Unity project (OC link)
2. Build for Windows
3. Run the build for a few minutes (1-2 is enough)
4. Watch it accumulate memory

Expected result: memory is properly unallocated
Actual result: memory is leaking

Reproduced with: 2017.3.0b2 (?), 2017.3.0b3, 2017.3.1f1, 2017.4.2f2, 2018.1.0b12, 2018.2.0a4
Not reproduced with: 2017.1.1f1, 2017.2.1f1, 2017.3.0a7, 2017.3.0b1, 2018.2.0a5, 2018.2.0b3

Note: the build for 2017.3.0b2 crashes so it is possible that the regression occurred there. Marked the regression point as 2017.3.0b3 since I could properly test it.
Note 2: the problem went away in 2018.2.0a5
Note 3: attached a couple of random builds for quicker illustration of the issue