Are you tagging elements with Material tags and think that the tag is not reflecting the correct value? - Just face it... you're wrong and Revit is right! Bwa ha ha ha ha.
Actually, you're probably getting a different element than you think when you go to place the tag. If, like my frantic designer, you are tagging very thin floor slabs (as finishes like carpet or tile), then just ensure that you temporarily hide from view the underlying floor slab (probably concrete) before you attempt to tag by material.
In our practice, we add thin floor slabs as finish materials on top of the structural slab and tag them, so the designer thought they were tagging a carpet, but Revit was secretly choosing the structural floor slab below. Using the temporary hide element from view tool available under the "cool shades" icon on your view control bar, will allow Revit to select your thin floor finishes rather than the main floor slab.
And Viola! Revit is once again playing nice! ... now back to work with you!
...Note: make sure your thin materials are indeed above the top of the structural slab to avoid this problem in the first place - My designer figured this out on their own and just reported back!