Personally, I get close. I had a 2.25-2.95yo in a MR, and he was 38lbs almost all of that time. I'd been certain he would outgrow by weight, but he got pretty close on height too by the end. He got up to 39.6, I was still ok putting him the seat but I weighed him frequently. I'm willing to weigh frequently to keep a child RF, especially if it's a matter of turning. In the end, I bought a RNSL, and while it's nice not to weigh, he still would have fit in that danged MR lol. (I bought it in May and planned to RF him through the summer -- we travel a LOT in the summer -- then pass down to the babe. I was praying he'd make it through summer, and he probably would have made it a full year.)
If I had not bought a new seat, I would have waited until the scale showed 40.1 to turn him. Incidentally, he's back down to 38.6, he gained that pound over Christmas, then lost it again, lol. But, I'm also comfortable knowing that there's some safety margin in there, and if a seat won't fail at 39.9 it won't fail at 40.0. I wouldn't knowing put a child in there above the limit, but I would *at* it. That's JMO, though, and I am not a tech.
One thought, you note 'when she was sick' -- how sick was she? If it's possible she had gone down a few pounds, keep an eye on her, she may regain that quickly.