I actually know the answer for this. I ran into the same problems a few years ago with a Samsung i760 - funny enough, eh? Maybe it's something about Samsung.. Don't have the problem now on my Touch Pro 2..
This is an explanation I drudged out of many calls to tech support and it's the only one that makes sense. So anyway, when you transfer TXT msgs between carriers i.e. AT&T <--> Verizon, there is a protocol that breaks up msgs to 160 characters automatically. However *within* Verizon they don't use this protocol, so you are now reliant on the phones "profile" within Verizon's systems and whether or not it supports EMS, and if my theory is correct, for whatever reason Samsung smartphones do not.. EMS stands for Enhanced Messaging Service and any text over 160 characters is considered EMS. Note this is entirely different from MMS which you use to send pics/video/etc. It's weird, most dumbphones work just fine, the problem seems to hit only on specific smartphones and I've had quite a few with Verizon over the years.
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This pretty much explains the behavior..