It's easy to avoid using u-P formulation in Mechanical because you have to use a Command Object under a body and set Keyopt(6)=1. However, if you use a hyperelastic material model, the solver might automatically set Keyopt(6) for you. You can stop it from doing that by setting Element Control to Manual.

I have never noticed a warning like the one above. You can chase it down by finding that element and see what kind of element it is.
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