Either sources or access transformers don't work in current version of ForgeGradle

When I update the buildscript to use another Forge version as dependency or different MCP mappings, Forge/MCP is updated after reimporting Gradle. When I run the game then, it all works fine. But when I try to inspect the remapped sources, it will instead show me a decompiled bytecode file. I can fix this by reimporting Gradle again, but when I do so, I can't run the game because the bytecode seems to be no longer access-transformed. The JVM comes up with several IllegalAccessErrors but when I inspect the source code it just shows me the transformed access modifiers there. This can again be fixed by changing MCP/Forge version in buildscript and then reimporting, but then the sources won't show up again.

Crash reports of those IllegalAccessErrors still seem to specify an AT service. Does this implicitly mean that FML's AT-config is missing?

ModLauncher services: 
	/eventbus-1.0.0-service.jar eventbus PLUGINSERVICE 
	/forge-1.14.4-28.2.1_mapped_snapshot_20190922-1.14.3-launcher.jar object_holder_definalize PLUGINSERVICE 
	/forge-1.14.4-28.2.1_mapped_snapshot_20190922-1.14.3-launcher.jar runtime_enum_extender PLUGINSERVICE 
->	/accesstransformers-1.0.1-milestone.0.1+94458e7-shadowed.jar accesstransformer PLUGINSERVICE      (what?!)
	/forge-1.14.4-28.2.1_mapped_snapshot_20190922-1.14.3-launcher.jar capability_inject_definalize PLUGINSERVICE 
	/forge-1.14.4-28.2.1_mapped_snapshot_20190922-1.14.3-launcher.jar runtimedistcleaner PLUGINSERVICE 
	/forge-1.14.4-28.2.1_mapped_snapshot_20190922-1.14.3-launcher.jar fml TRANSFORMATIONSERVICE 

Also, when Forge or MCP updates, Gradle shows the following log messages which don't appear when reimporting again:

[09:15:00] [main/INFO]: Writing debug log file accesstransform.log
[09:15:00] [main/INFO]: Access Transformer processor running version 1.0.5+4+02b7b69
[09:15:00] [main/INFO]: Command line arguments [--inJar, /Users/.../.gradle/caches/forge_gradle/minecraft_user_repo/net/minecraftforge/forge/1.15.2-31.1.0/forge-1.15.2-31.1.0-injected.jar, --outJar, /Users/.../.gradle/caches/forge_gradle/minecraft_user_repo/net/minecraftforge/forge/1.15.2-31.1.0_mapped_snapshot_20200322-1.15.1/forge-1.15.2-31.1.0_mapped_snapshot_20200322-1.15.1.jar, --logFile, accesstransform.log, --atFile, /Users/.../Documents/Programming/Minecraft/TheModernity115/build/_atJar_1043152830_/parent_at.cfg]
[09:15:00] [main/INFO]: Reading from /Users/.../.gradle/caches/forge_gradle/minecraft_user_repo/net/minecraftforge/forge/1.15.2-31.1.0/forge-1.15.2-31.1.0-injected.jar
[09:15:00] [main/INFO]: Writing to /Users/.../.gradle/caches/forge_gradle/minecraft_user_repo/net/minecraftforge/forge/1.15.2-31.1.0_mapped_snapshot_20200322-1.15.1/forge-1.15.2-31.1.0_mapped_snapshot_20200322-1.15.1.jar
[09:15:00] [main/INFO]: Transformer file /Users/.../Documents/Programming/Minecraft/TheModernity115/build/_atJar_1043152830_/parent_at.cfg
[09:15:00] [main/WARN]: Found existing output jar /Users/.../.gradle/caches/forge_gradle/minecraft_user_repo/net/minecraftforge/forge/1.15.2-31.1.0_mapped_snapshot_20200322-1.15.1/forge-1.15.2-31.1.0_mapped_snapshot_20200322-1.15.1.jar, overwriting

It's pretty weird and it only happened to me in the last few weeks. Any support on how I can solve this? Issue happens in:

• ForgeGradle 3.0.159+
• Gradle 4.9
• IDEA CE 2019.3
• OSX 10.14

Edited March 26, 20205 yr by RgSW