It's a Mad, Mad, Mad Hotel - S1-E12
Other mistake: In the episode "Its a Mad, Mad, Mad Hotel", when Esteban has his dream of becoming rich and London becoming his "Bell Girl", he calls the hotel "Hotel Esteban, but London's uniform still has a "T." When it cuts there is an "E" on the front desk.
Answer: Keep in mind, Muriel wasn't very popular. So the writers/directors must have thought it'd be a good idea to let go of the character. No one seems to be complaining.