Same problem here... Bought the Keystep 37 last year, and on the first weeks i noticed the problem. At the time, since i had only used the controller for a very short time, i thought i was using it wrong or something like that. Read the manual but found no solution... searched online and found no one else with the same problem... i ended up stopping using the sequencer mode.
Today i stumbled with this problem again so i made another search online and found this post.
Im kinda relieved to know im not the only one with this problem, but also very frustrated, because its such a a fun and amazing feature, and it's completely unusable. Hope a firmware update can fix the problem soon!
By the way, playing with the bug i found out the random notes are not random at all!!! It seems to me like its an old sequence that's leaking out from the rested notes. Since it only happens (at least to me) when setting the sequence length up front (all rests), or when creating a step sequence that contains rests. If the sequence has no rests the bug just doesn't happen.
If you want to try to reproduce the error, create a sequence of some fixed length, press play to real-time record and hold a single note. If you dont release the note when the sequence loops the new pattern starts to appear.
Would love to know if its the same problem we are all experiencing.
UPDATE
Used the midi center to delete the saved sequences and the problem dissapeared!!!
Sadly, the problem reappears after creating a new sequence. And deleting the sequence form the keystep directly (shift + octave buttons) doesn't seem to truly delete the sequence.
Don't know if there is some other way to delete a sequence directly from the keystep
UPDATE UPDATE
found a way to truly delete the current sequence from the keystep, solving my issue.
On a running sequence, hold the rest button until the sequence loops. This will fully clear the pattern, and the ghost patterns wont appear anymore on overdub recordings.
The issue was that the other ways to create a new sequence didn't thruly delete the previous sequence, making it leak to the new sequence...
Hope this helps somebody!!
P.D: Sorry if my writing is a bit off. Im from Argentina, so English is not my first thongue.
Good luck to you all!