I think that breaks for over a year could potentially be considered on a case-by-case basis however for Stacey I would definitely think she should not be marked as having left in 2019 considering she still played a major role in storylines (constant referrals to Stacey, "when Stacey comes back" etc...), did some voice work for the show during her break (I believe in May of this year?) and I think even popped up for a few days at one point so to consider her as not part of the show would be preposterous in my opinion.
If there is a situation where the actor is on a break from the show for over a year and is for all intents and purposes plays no role in the show (unlike Stacey) then I am of the opinion that maybe in that case we could consider marking them as absent in their duration however. This should be really rare though, the only example which comes to mind for me is that of Ben Mitchell, who left with some finality (although I believe it was always intended to be a temporary break for the character) in January of 2019 but returned only 8-9 months later, albeit under a new actor. I would expect that we would have marked the character as absent from the show during that time although our hand was forced to not do that because it is generally accepted that something like "2014-2019, 2019-" should not be used for durations so we ended up having to stick with "2014-".
Aside from rare exceptions like that which I listed however I am of the opinion that breaks should never be included in durations.