I have never had a problem with this given that the leftmost column of Score manager has the correct string instrument selected, the Human Playback setting is correct (I would think most non-custom HP styles would work), and the arco expression is one provided rather than a custom addition by a user.

Now this is a different situation from starting playback partway through a composition, given that pizz and arco do not "chase" through the program. If you stop within a pizz section and then start in an arco section, the pizz will likely persist.

J ADRIAN VERKOUTEREN: Thanks for replying!
I already assumed that this problem could occur when starting playback partway through a score. However, the same problem arises when starting playback from the very start. I tried switching off Human Playback: no change. What bothers me especially is that this problem does not occur when the score instruments are in SmartMusic SoftSynth setting.

I also found out that, when trying to start playback a score part (Score Manager in SmartMusic SoftSynth setting), Finale crashes. When in Garritan Instruments settings, Finale does not crash but again it doesn't pick up arco after pizz. ??

I guess I'm stumped. You might try checking the Human Playback Preferences to verify that the keyswitches have not been inadvertently changed. Otherwise you can use the "Submit a request" link above (if you have not already) to send your errant file to MakeMusic support for their analysis.