Yes it is a force.. The runner must return to the 1st and may not advance until the tag up.. If she can NOT advance until the tag up please explain how it is not a force.. I understand its not a force out, as in the runner advances due to a hit ball. I did not cite it as a "non-verbal appeal" However the runner is forced back to first for a tag. That was my point.. My bad for not citing the correct nomenclature..
In addition you may leave prior to the fly ball being caught, again the tag is what is important.
Take a look at this scenario. Runner at 3, less then 2 outs, bomb is hit to the outfield, say right field. Girl jumps on the release. Certainly there is enough time, after the catch, to tag up at three and go home. In no way is runner "OUT" because she "left the base before a fly ball was caught".. Again the tag is the key here... After the relase the runner can have cake and krumpets in the base line, as long as she is back to 3 BEFORE the defense throws to 3. That's my point on saying the runner is "forced".. I should have had said the runners forced to tag up...