Add "debug dump" to your connection parameters, e.g. after noauth.
After connection attempt fails, look at journalctl as you did before. There may be a bit more detail, but not if the hangup happens quickly. Also look at syslog. pptp is so old that's where it logs. pptp has a debug flag for additional output to syslog. It's good you've identified changes in ppp that have caused this. That has happened before. A bisect of those changes may help. Or a scan of the changes from 2.4.9 to 2.5.2, paying attention to anything to do with the pty option or implementation. I've counted the commits, and it's a big job, so a bisect would be easier.
