It depends if it's open or closed cell foam. If you're insulating the walls of your crawlspace, open cell foam is still recommended on wood because it can breathe

don't they usually have a gap for moisture/condensation to escape?

if you spray foam the rafters they are encapsulated in foam.

Not in any situation I know of. 3 sides are in foam, the 4th is attached to plywood and the other side of that is the outside. That is enough to let water out. It is slow, but we don't need it to be fast.

The important part is there is exactly one barrier that water cannot get through. Spray foam is generally good. there are different spray foams as well. Open cell and closed cell work completely different.

Not if that barrier traps water against regular wooden framing members

Water will move through the wood framing to the other side and out. It takes a little longer, but we have plenty of time. It is when the water cannot get out either side that you have a problem.

I am hopeful that the +100r old timbers haven't suffered, unfortunately it's becoming an issue for mortgage companies, who seem to be refusing to lend as a blanket policy. We have a roof guy coming tomorrow to check out the situation though so fingers crossed. We're prepared to have it removed if necessary but aren't relishing the extra cost or the upheaval of clearing the loft.

Foam on timber is a bad idea, for those talking about open cell vs closed cell foam in this thread, beware, both are bad for timber. Im renovating s Victorian property at the moment and when I run the simulations on condensation and vapour pressure etc etc… in several colder climates the system even with open foam accumulates enough moisture content in the wood to rot it. A large part of that is the cold bridging effect generating liquid water close to the wood which it soaks up much faster than it can release during the summer. Please always run a simulator for your climate before allowing modern materials near timber and or old properties.