How to reproduce:
1. Extract the "repro.zip" folder
2. Change path variables according to your machine in "repro.sh" script
3. Execute the "repro.sh" script

Actual result: Unity loses reference to Assembly2.dll and tests fails with compilation errors.
Expected result: Unity keeps the reference and tests are executed.

Reproducible with: 2017.4.33f1, 2018.4.11f1, 2019.1.0a1.
Not reproducible with: 2019.1.0a2, 2019.1.14f1, 2019.2.8f1, 2019.3.0b6, 2020.1.0a8.

Notes:
- With 2017.4.33f1, on the first run the assembly is still missing but the tests are executed.

---------------------------
Fixed in 2019.1.0a2, 2018.4.13f1.