It may do. Russia's deterrence doctrine allows first use if there is "
aggression against the Russian Federation with the use of conventional weapons when the very existence of the state is in jeopardy." (
https://sgp.fas.org/crs/nuke/R45861.pdf).
The results of the referenda, hovered bogus that may be, now clearly place occupied territory within the Russian Federation. Any attempt to recapture them by the Ukrainian forces can now be considered (in the eyes of Russia and its allies) as an attack on the Russian Federation. I believe the reason the referenda were called so quickly is a desperate attempt to keep hold of the occupied lands following the failure of conventional forces in that endeavour.