Advice needed on shower curb gap with void pls

Hello,

Looking for some advice / opinions on how to address these gaps with deep voids under my shower curb. My builder recently did some work on my shower and replaced the curb. It now looks to be raised slightly with a gap that is a deep void behind (no backing).

They seem to think to solution is to simply silicone the void. My concern is that there is no backing and if / when the silicone fails it will be a bad.

Apparently I am making a "big deal" about this.
What are your thoughts? What is the right way to address this?

Thanks in advance.