I have a cope theory for the ending:
In order to save Mari's life, Lara literally gives over her true love. She stops loving Mari because of this and that's why she's so disinterested in the end (also, the mermaid ring that before turned red when close to your true love is now blue. In episode 1 you can see it's red on the first moment Lara and Mari meet).
Of course, this only works if they announce some kind of continuation to actually explain and deal with this because if this was somehow the intention they didn't explain correctly.
Being realistic I think Anna Kawahara is just a homophobe that's always been very explicit how much she dislikes Mari as a character. I hope she would never get another job in the industry.
I don't think a continuation is likely here unless Koide is just as dissatisfied with the ending and is willing to fight for one.