Don't quote me on this, as I might get one or two of the specific details wrong, but if I'm not mistaken, this is due to how the note loading works. Not all of the notes in a sequence are loaded at the same time. In order to allow for sequences to run better on computers, only the notes that are within a few measures of the measure you are visually on are loaded in. Please note that the positions of the notes are based upon where the beginning of the note is, as that is when the sound is played, so the position of note 7 measures long that starts in the 1st measure is still considered to be in the first measure, meaning that if you move over to measures 4-6, it will disappear before the note ends.
However, please note that this only affects things visually, the audio still plays out like normal, and for instruments that cut off at the end of the note itself (the 8-bits, flute, and e. guitar), they still cut out at the correct time.
Except for 8 bit, flute, and electric guitar, the length of the note doesn't affect the sound when played on the website. It will if you export a MIDI file.
