How to reproduce:
1. Open user's attached project (Libmonoposixissue.zip)
2. Make sure 'SampleScene' is open and enter Play mode

Expected result: Deflated string array length is printed to the console
Actual result: "DllNotFoundException: libMonoPosixHelper.so" is thrown to the console

Reproducible with: 2019.3.0a6, 2019.3.1f1, 2020.1.0a23
Not reproducible with: 2017.4.36f1, 2018.4.17f1, 2019.3.0a5
Regression from 2019.3.0a5 to 2019.3.0a6

Full error:
DllNotFoundException: libMonoPosixHelper.so
System.IO.Compression.DeflateStreamNative.Create (System.IO.Stream compressedStream, System.IO.Compression.CompressionMode mode, System.Boolean gzip) (at <e304493dc8584f28b068435a506526d3>:0)
System.IO.Compression.DeflateStream..ctor (System.IO.Stream compressedStream, System.IO.Compression.CompressionMode mode, System.Boolean leaveOpen, System.Boolean gzip) (at <e304493dc8584f28b068435a506526d3>:0)
System.IO.Compression.DeflateStream..ctor (System.IO.Stream stream, System.IO.Compression.CompressionMode mode) (at <e304493dc8584f28b068435a506526d3>:0)
(wrapper remoting-invoke-with-check) System.IO.Compression.DeflateStream..ctor(System.IO.Stream,System.IO.Compression.CompressionMode)
DeflateString.Start () (at Assets/Runtime/DeflateString.cs:16)

Notes:
- The file when looking at it from a non-Unity context, seems to be a part of 'mono-devel' package, installing it should fix the error.