I have this issue arise with catasetinae actually with a cycnoche at mo to be specific. All I can reccommend is supporting/ staking the new PB and allow time for flower to arrive, display and leave and treat as normal during dormancy. The yellow leaf and drop indicates to me to reduce watering and adhere to guidelines regardless and if need be treat as an isolated case. The fact that its a large PB is a bonus and would definetly not risk the rot creeping in. After dormancy the new growths will aid in anchoring itself more securely. Suprisingly it can be as little as 24 hours for rot to accept an invite and begin making your life a misery. Begin procedures as normal for dormancy but keep well lit asit may throw out a spike at anytime. my opinion anyway!