I hadn’t checked for at least 2 days, but as I look very late on a Friday afternoon, I see the missing episodes, both formats. (Legacy).
Another way it could have been PBS’ fault is if the Comcast On Demand implementation is perhaps somewhat inelegant. Perhaps it requires two copies of the exact same video file, each associated with separate metadata (cost, expiration), and PBS perhaps only supplied for the for-pay version initially.
Comcast isn’t transparent on these matters. That wouldn’t much matter if Comcast reps were more responsive when such issues arise. When you combine the lacks of transparency and communications, we users are truly in the dark as to whether our complaints should go to Comcast, or to the network provider. Oh well...