Next season Mbappe will be on a free transfer after his contract with PSG ends, of course this is the right time for Real Madrid because this season the transfer problem has not gone away because PSG wants a high enough price so there is no meeting point for these two teams.
While Bellingham was really impressive at the start of this season, he always scored in the first 3 games in a row and even now he is currently the top scorer in La Liga, this is quite impressive for a midfielder.
I don't know for sure if it's because Bellingham is in top form that Real Madrid can relax enough even if they don't get Mbappe.
I don't see any problem in this, Bellingham scores and it's great for Real Madrid, I doubt that they are now looking for someone else to sub-position the striker. If Mbappe becomes a free agent next season, then Real Madrid will certainly not refuse to take him to the team, but everything is always not easy with Mbappe, and he can still change his mind, extend the contract with PSG, or whatever happens, Real will not wait until next summer to find out, but if Mbappe comes as a free agent they will certainly take him to the team.