The "BOM seen flag" is relevant to a single stream. Therefore it should be reset before decoding a new stream, e.g.

decoder = new TextDecoder('utf-8'); // BOM seen initially unset
decoder.decode(buffer1); // so unset when decode begins
decoder.decode(buffer2); // needs to be unset before this
decoder.decode(buffer3a, {stream: true}); // needs to be unset before this
decoder.decode(buffer3b); // but not before this

Step 1 for decode(input, options) should be changed from:

1. If the streaming flag is unset, set the encoding state to the default values of the encoding's decoder's associated variables and empty the stream.

to:

1. If the streaming flag is unset, set the encoding state to the default values of the encoding's decoder's associated variables, unset the BOM seen flag, and empty the stream.