Single ended to differential amplifier workings
Well, this is exactly what should happen ! The reason is as follows:

Your input signal is a sinewave between 0 - 2.5 V

I could also say that your input signal is a 1.25 V (2.5 V /2) DC voltage + a sinewave with an amplitude of 1.25 V.

The thing that confuses you is the 1.25 V DC !
It is converted to differential as well, resulting in the DC voltages you see at the outputs.

If you want to prevent this you need to make that DC voltage at the input 0 V. This might be impractical.

Fortunately what you can also do is shift the DC reference voltage of the single to differential converter. I see that is has a Vocm input, if this is a common-mode level input then you could apply 1.25 V DC to it (the DC voltage I mentioned above) then the single to differential converter will consider 1.25 V as the commonmode level and the outputs will get the same DC voltage.