Forget prog change and the MD just for a minute
Does the OT change pattern straight away when you change pattern? If not there’s the issue, change the OT pattern settings and it will fire that to the MD as requested.
I screwed up and edited my post as I was talking arranger, ignore me I’m waffling.
To do this that you desire: make sure you are not in record mode (no red light) and hit FUNC+BANK, then arrow across to the option that says PAT LeN, this tells the pattern to change how you decide. I assume it reads as PAT LEN on yours? That means it will change pattern after pattern length is complete. You will want to lower this by using the LEVEL knob, bear in mind that this has to be done on a pattern by pattern basis, so if you’re in pattern 1 and you make this setting but decide you want it for a million other patterns then you have to change those million other patterns, if you only want the Transition pattern to react immediately just change that one. Oh, the lowest it can go from memory is 2/16 which is basically a trig in real terms so it’s pretty immediate
All this is in your manual too I know I know hahaaaa but it’s useful so I read mine all the time for fun kidding but it’s good to know it.