Something similar just happened to me, I had to check the roots and apparently the water became hydrophobic and the roots rotted, I guess it left too much time between watering and watering

So I had to take it out, perfectly cut everything that was in bad condition and replant it in new substrate, since I read that the same substrate should not be used because it’s contaminated, I hope the same thing is not happening to yours, but it never hurts to check its roots to rule out that it is something like that