OK, I'm going to throw this one out there cuz the Wife and I were just talking about it.
Megan Boon is PREGNANT and this time it's TWINS, which is why she hasn't been on the show in recent months!! Think about it. They have ONE child and if like most couples wanted more than one so I'm betting this is the answer.
I thought about that possibility rather than the possibility of she working on another project,
A maternity leave sounds reasonable to me, that the producers would be willing to accomodate her, and still keep the Liz character in the shadows so to speak.
On the other hand, I don't believe that the producers would be so accomodating if Megan Boone told them something like: "Hey chiefs, I won't be showing up for work in the next months because I'll be working on another TV series, but keep my character alive, please and I'll be back as soon as I finish that other project"
Having said that, I don't think she'd be back soon in case the maternity leave is the reason she's been absent.