For some reason this doesn't happen in /_rtform/ , but if I copypaste the output of /_wikitext/ into /_html/ (which is closer to what happens in VE in practice), I can make it happen.
Wikitext input: <pre>\n\n\nFoo</pre>
HTML output: <pre data-parsoid="{"stx":"html","tsr":[0,5],"dsr":[0,17]}">\n\n\nFoo</pre>
HTML input: (same)
Wikitext output: <pre>\n\nFoo</pre>
Version: unspecified
Severity: normal