Running PackageAccessFixer before Mixin causes warning messages
When a Mixin using an anonymous or otherwise package-private target (for example @Mixin(targets = "net.minecraft.util.Language$1")) is loaded in a development environment Mixin outputs a warning saying @Mixin target net.minecraft.util.Language$1 is public in ... and should be specified in value.
Running PackageAccessFixer before Mixin makes these classes public, which makes Mixin believe that they could have been specified in value.
In dev there is no real way to prevent this. All I can say is that this is probably intended.
Based on messages sent by modmuss50 in the discord, this is a bug that is intended to be fixed at some point, as such this issue report shouldn't be marked as wontfix.
[3:21 PM] modmuss50: We need to fix that [3:34 PM] modmuss50: These warnings are technically a loader bug