It is possible your score has some sort of corruption that is causing problems; we'd need you to attach your score and steps to reproduce the problem in order to say for sure.
My guess, though, is that what you are seeing are not "extra" rests but are just the normal rests that are needed to keep the measure full. For example, a measure in 4/4 time starts out with a full measure rest indicating four beats of rest. If you replace that rest with a quarter note, that leaves three beats unaccounted for, so the corresponding rests are generated to keep the measure full. Enter another quarter note and it will replace the generated quarter rest, etc. All so you don't have to manually keep track of how many beats you have added to each measure or worry about under- or over-filling it.
For more info, see the "Note input" section of the Handbook.
Comments
Take a look at https://musescore.org/en/handbook/voices#delete-hide-rests
'rests in voice 1 are permanent. However, they can be hidden'
Explain more and attach here your score
It is possible your score has some sort of corruption that is causing problems; we'd need you to attach your score and steps to reproduce the problem in order to say for sure.
My guess, though, is that what you are seeing are not "extra" rests but are just the normal rests that are needed to keep the measure full. For example, a measure in 4/4 time starts out with a full measure rest indicating four beats of rest. If you replace that rest with a quarter note, that leaves three beats unaccounted for, so the corresponding rests are generated to keep the measure full. Enter another quarter note and it will replace the generated quarter rest, etc. All so you don't have to manually keep track of how many beats you have added to each measure or worry about under- or over-filling it.
For more info, see the "Note input" section of the Handbook.