The ISTD is almost always writing foxtrot figures with the last step actually being the first step of the next figure. But I don't think they ever say that this is what they're doing. Of course you figure it out soon enough when you realize one figure ends on the same foot that the next one is supposed to start. However, I'm writing to note that the IDTA does not do this. It's been a while since I looked at the IDTA books (slightly older ones, I haven't gotten the latest revisions yet), but I am fairly sure that they do not write foxtrot figures like the ISTD does. When it ends, it ends, none of this last step of this figure = first step of next figure business. So you could say that instead of modifying ISTD definitions, people are using the IDTA definitions instead.
It's not unusual for figures to cross musical bar lines. Basic American foxtrot with its SSQQ rhythm does it all the time. So does east coast swing. Around here people dance 3 count hustle, dancing four steps in three beats to music in four -- you're off the musical bars 3/4 of the time. Quickstep is often in a SQQS pattern putting it in the same category as the SSQQ rhythm -- six beats for four steps over 1.5 bars. Probably others, but I'm not taking the time to sit and think about them. :)