How to reproduce:
1. Open the attached "1250707 repro" project
2. Open the Build Settings from File > Build Settings
3. Make sure that Development Build is set to true
4. Build and Run the project

Expected result: an error is thrown stating that "AlwaysCondition" class from "Conditions.cs" was stripped due to managed code stripping
Actual result: an error is thrown suggesting that #ifdef UNITY_EDITOR has been used in a section of serialized code

Reproducible with: 2019.4.2f1, 2020.1.0b14, 2020.0.a16
Could not test with: 2018.4.23f1 ([SerializeReferece] doesn't exist)

Notes:
- The issue reproduces with Mono and IL2CPP Scripting Backends
- The issue on this specific project reproduces with Medium and High Managed Stripping Levels
- #ifdef UNITY_EDITOR isn't used in any of the Scripts making the error even more confusing
- The full error states: A scripted object (probably QuestSettings?) has a different serialization layout when loading. (Read 116 bytes but expected 164 bytes)
Did you #ifdef UNITY_EDITOR a section of your serialized properties in any of your scripts?