Oh man do I feel your pain! I just jumped into the same boat as you.
Today I took out my Kala acacia concert uke and played with her a bit and as I was just giving her a wipe down before putting her away, I noticed a very fine hairline crack on the soundboard. My heart just hit the floor. I'm not 100% certain on this but when I first started playing it before I noticed the crack, I did sense that it sounded a little more "open" but my tuner said it was fine. I thought it was because I was playing the tenor just before playing it.
The RH in the room with my ukes is around 45-52%. The odd time I've seen it hit 42%. The local store has been out of stock of the Oasis ukulele humidifiers so I placed an order with Eldery for some strings and more humidifiers but they're still in transit. Needless to say, I should have just done a homemade one to buy some time but I figured since the RH is usually around the 50% mark, I should be okay for a little longer until my Elderly order arrives. EHHHHH! Boy was I wrong! Dang, I spent 13hrs buffing that baby to a glossy shine too!
My Kala acacia tenor has a humidifier and sits in a hard case. Avg RH in the case is around 50-51%. Now I'm scared 'cause I'd hate to see a crack in my tenor. I'm thinking of adding an additional humidifier in its case to be safe. My concert is in an extra thick padded gig bag. Does a hard case retain more humidity than a gig bag? If so, I might be out to get a hard case for my concert... a little too late.
When I get my Kamaka, I'm gonna humidify the heck outta that one!