I would expect that result. Look at the Tempo map when you use the Fill option.
It creates a lot of entries that MIDI can handle much better than audio (RTs).
Asking Audio to stretch for that many Tempo map entries is pretty daunting.

I don't think the RT control is quite there yet for this.
You are cutting the tempo almost in half over 5 or 6 bars. And the Fill feature adds a LOT of gradual tempo changes in those bars.

Like I said; I don't think the RTs are that advanced quite yet.

I do not work here, but the benefits are still awesome
Make your sound your own!