According toCinema Blend, Boyett recalled these conversations with the actresses.
Mary-Kate said, ‘It would have to be me because Ash doesn’t want to do it.
But the timing is so bad for us.'"

The series did what it could to explain away the twins’ character, Michelle Tanner’s absence.
Later, he says, “It’s so nice to have three daughters again.”
What happened to Michelle?

The family worries about her getting her memory back (perIMDb).
A Redditor discussed the theory that Michelle died after the series' finale we saw.
“There are two theories about Michelle Tanner.

The first is she is dead,” wroteReddit user prankerjoker1.
“The cast always talks about her in the past tense.
The theory is she died.
Possibly from the concussion she got from horseback riding.
In the last episode of the original show, we saw that she got her memory back.
[This] was the end ofFull Houseas we knew it.