Sounds buggy, but what happens when you ignore the digitakt and hit start and stop from the syntakt transport only? What happens when you disconnect the pocket piano from midi? What happens when you disconnect the digitakt and try with just the pocket piano? What happens if you set up a brand new project with the same 3 pieces of equipment, with no other information copied over, and try a basic test of the same routing to see if it happens again or not?
Just a few things you could consider trying out to find out if one piece of gear in the mix seems to be causing the issue, if it’s specific to this project, or if it’s a bug. If you can figure out a condition under which it doesn’t occur, then when it does it will be easier to figure out why. If it’s only when outside transport is initiated, maybe try using a different piece of hardware (other than digitakt) as transport and clock to see if the results are the same.
And if it is a bug, post it in the syntakt bugs thread as well.
Good luck!