Using /playsound with an invalid <source> causes the client to drop out of the world with the following message:

Internal Exception: io.netty.handler.codec.DecoderException: java.lang.IndexOutOfBoundsException: readerIndex(30) + length(1) exceeds writerIndex(30): UnpooledHeapByteBuf(ridx: 30, widx: 30, cap: 30)

One can immediately rejoin the world.

The following command WORKS:
/playsound minecraft:entity.cow.ambient master @p ~ ~ ~

The following command CRASHES:
/playsound minecraft:entity.cow.ambient mastr @p ~ ~ ~

This happens both if the command is entered in chat and if the command is run via command block.

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Additionally, the old style commands do not crash, but there is one related problem. Running /playsound minecraft:entity.cow.ambient @p returns the following in chat...

An unknown error occurred while attempting to perform this command.

There is no crash. Meanwhile, /playsound minecraft:entity.cow.ambient @p ~ ~ ~ fails with the expected

That player cannot be found.