I came across a second UE that behaves in the same way…
In the first case, it was possible to configure the UE to not do SMS over IMS at all.
In the second case, the UE manufacturer have not exposed any way to prevent it.
The manufacturers of the UE are different, but in both cases, they are using Qualcomm chipset (I believe it is even the same chipset) making me believe that this behavior is rooted in Qualcomm supplied code.