I only just found that the way I've been registering log processors via AutoConfigurationCustomizer is seriously flawed, and worried other people may be doing the same:

AutoConfigurationCustomizer.addLoggerProviderCustomizer(
            (builder, otelConfig) -> builder.addLogRecordProcessor(...))

Since this will register the log record processor after the BatchLogRecordProcessor, any mutations made by the log record processor may not be exported, although 99.9% (~) of the time the mutations will be exported since they will be made prior to the batch being exported.

One (non-obvious) workaround is to leverage AutoConfigurationCustomizer.addLogRecordProcessorCustomizer().