When you build an Android app Java code is first compiled by a regular Java compiler into class files and then converted to a DEX file using the R8
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Kotlin code also compiles to Java bytecode because the JVM language.
Therefore, if you write an Android app in Kotlin without thinking about JMM, it is possible to Java bytecode, which is incorrect opisyvaet your intentions, and it will be broadcast in an incorrect Dalvik bytecode and behavior work appropriate.
In General, you need (in my opinion) to bother with the study of the JMM