BeatStep Pro standalone - merge box stuck notes

Hi everyone. Experienced midi user here, so i’ve tried most common solutions (cables, ports etc)

I work for quite somebtime, with a perfectly functioning, updated to the latest firmware Beatstep Pro, in a studio environment along with several hardware synths, a reliable midi set up, including a multi prt interface and midi translator and all work as expected for whatever i want to achieve.

Lately i made a smaller live set up incl. the beatstep pro.
The set up is pretty simple.

On the output side i have a key controller and the beatstep pro going to the ins of a midi man merge 2x2 box.
The merge box is quite simple and works perfectly well with 9v and the cables are perfect.
The midi stream on the merge outs is as expected (monitored with midi ox).
All notes on and off messages of both output devices combined. No other “rubbish” messages.

As it’s normal whenever i use the drum sequencer, along with extra sequencer notes, and notes played on the keyboard controller the monitored stream flow is fast.

On the input side the merge box has two outs (working like a simple thru box). So i take one out to a trustworthy old alesis drum machine (used in my case as a drum sound module), and the other out to a modern dexibell piano-key module.

Both machines have thru ports but i don’t use them so far, since i’m facing a problem with just the current set up.

What the problem is?

If the stream doesn’t include the drum sequencer of beatstep pro all work well.
But whenever the drum sequencer starts playing, especially with quicker notes, and at the same time i play the key controller, the piano module starts giving stuck notes.

The dexibell piano module has midi filter for both transmission and reception. I ‘ve filtered everything except midi notes reception.
And of course all beatstep pro sequencers (even the knob control section) and the key controller transmit in different output channels.

Should i assume that it’s a problem of the merge box outputs (thru) and that the results would be better with a good thru box?
Or should i forget the idea of input merging alltogether? But that absolutelly creeple my set up.
The m audio merge 2x2 is considered a reliable standalone merge box.

I intend to stick on standalone boxes and set up.
Any tested suggestion?
Is there something i could do on the beat step pro to make things work better? I know i could forget about the drum sequencer in that set up, but i really like it, so i need it.