Wightlink tells us
Checking in
Foot passengers need to check in 15 minutes before departure.
Find out what it’s like to travel with us on foot
And I suspect that means that only trains timed to arrive 15 + 5 = 20 minutes before the ferry leaves will be offered on journey planners.
I also suspect that the same connection time from A to B mode and B to A mode is used in the railway's planners ... so the ferry arriving at 18:07 won't show any onward connections until a train at 18:27. Ergo - does not connect with 18:24.
If anyone knows better, please correct me.