User blog comment:Watermaiden15/Lucy's new bathtub subtly hints that Fairy Tail isn't ending yet/@comment-2214433-20161220053525/@comment-7418318-20161220063128

It's also a spoiler to anime-only viewers in general, since the anime stopped before Alverez and Brandish were introduced.

That aside I wouldn't take anything that happens in OVAs, Omakes and Movies too serious. We had several Omake and OVA that clearly can't exist in the actual series canon (413 days for example), so the Hiro Mashima and/or the animators taking liberties with the design of a bathtub is almost assuredly just that: liberties in how to draw it without any deeper meaning.