It is a matter of interpretation whether separable verbs are two words or one. Viewed in one direction, this means that they often form a unit of meaning but can nevertheless be broken up in the syntax of the clause. Conversely, a verb with an addition that is very close can be viewed as "one word" to some extent if you want. It's not so clear what you mean with "word" in the context of particle verbs, after all.
This ambiguity between one or two words arises especially with additions to a verb that denote a result or a direction (the two seem to form a unified category to some extent), or which specify how the event unfolds through time (aktionsart), e.g. "los" (to start), or "weiter" in the temporal, instead of spatial, sense (to continue).
From that perspective, the reason why something like "weiterverlaufen" is not listed in dictionaries could be that it is too regular. The same thing as with compounds. The contribution of aktionsart markers or result or direction is indeed more predictable than that of the particles that go back to locative prepositions ("an-, auf-" etc).
For another thing, the spelling as one written word is often meant to reflect intonation, i.e. the absence of a new stress on the base verb. I would usually say "WEIterverlaufen" instead of "WEIter verLAUfen". But that's now "one word" in the sense of intonation, and this is a third category besides a unit of meaning or a unit of syntax. So all in all, "word" is an ambiguous thing that exists at different levels of grammar, and the different kinds of division do not always coincide.