Don nailed it.
If I wrote a check expecting a key member of management to be part of the team and devote their efforts to building the business and creating value for the foreseeable future and then found out they knew they were not going be part of the team or significantly constrained in their commitment I would be very unhappy. If I were told there is a plan for a key manager not to be involved past a certain point or to substantially modify their role either temporarily or permanently I would have factored that into my decision and be OK when that change went into effect (but it may have effected my view of valuation, risk or even terms such as wanting the right to redeem some of their shares if the person did not come back to work).
I suspect an attorney would say not disclosing could be a material omission but I would use a different approach. If you were looking to for space would it make a difference to you if the lease was for two months rather than five years and if so would you want to know? If you were carefully interviewing nannies would it bother you to find the perfect nanny who started last week had been planning to move to Europe next month and hadn't told you? Would you want to pay a large initiation fee for a sports club and then learn three weeks later they were shutting the facility for six months for floor to ceiling renovations?
Life happens - people get sick, spouses move, they even get pregnant. The difference between those situations and yours is that you know and it may effect an investor's decision.
By the way: Congratulations!