I was up on the ladder and I saw that my soffitts were covered again,
and this time I noticed that so were my neighbor's , even though he
doesn't have a roof fan.

There was a lot of something in the air during the spring, milkweed
maybe. It was inches long and stringy and white and floated. I think
this was unusual, but regardless, our soffitts are covered with this
or a lint-like substance.

Also my fan has been running much later, hours, than it ever did
before.

If you depend on your soffit venting to keep your attic cool, check
them every few years. I have a two story house and I can't tell from
the ground if my soffits are clogged or not.