The thing is, 303 style synths with MIDI usually handle ties and slides with overlapping notes - if you let a note sustain a little bit into the next note, the synth will interpret that as either a tie (if they’re both the same note) or a slide (if they’re different). So basically the same as legato mode in many other synths, hopefully fine tuned to be as close as possible to the original 303 sequencer behavior. I’ve been using n0nx0x2 since 2014 or 15 so I could be misremembering, but I’m pretty sure the original firmware works the same way.
Since the Octatrack’s MIDI sequencer tracks aren’t truly polyphonic, it isn’t very good for that approach, because you can’t have overlapping notes on a single track. I don’t know if there’s a workaround.
Maybe try an alternative OS? I liked MarOS a lot before I upgraded the CPU and switched to n0nx0x, but I can’t remember if it has any options for adjusting the way it responds to MIDI. At any rate, they’re all free so the only thing you have to lose is a bit of time.