Yep, this. There is no bug, this is actually their name.

https://unicodeplus.com/U+2588

The character █ (Full Block) is represented by the Unicode codepoint U+2588. It is encoded in the Block Elements block, which belongs to the Basic Multilingual Plane. It was added to Unicode in version 1.1 (June, 1993). It is HTML encoded as █.

There is a separate block character, usually a perfectly square one, that is shown when your system cannot display a unicode character, however this block character is rectangular.