If you open an old post in Gutenberg it will put the contents in a classic block. If this classic block contains a <pre> section and you then convert that to blocks it removes the new lines in the editor.

HTML is:

<pre>line1
line2
line3
</pre>

From the block options pick 'convert to blocks' and get:

Note that this is only in the Gutenberg editor, where the code is:

When viewed in the post the <pre> is correct:

Expected behavior

Either the <pre> conversion should add the <br/> tags that occur when created in Gutenberg, or the preformatted block should insert breaks in the display when a new line is encountered.