Only one locomotive or digital accessories must stand alone on the track when you program and change value.
In this case you did changed all locomotive when you did changed the Brawa locomotive.
I show this draw picture one exemple what you should have done by isolated main track and programming track.


If you want to reset your locomotive i recommended you one locomotive a step in time.

Thanks Goofy and Jean,

There was enough information in your replies to sort the problem.

Yes, the Brawa loco was identified/programmed separately - no other locos were on the track.

I found that I only had to turn the Marklin loco lights on and off or do something similar to get them to register correctly with the MS2 before driving the Brawa loco and that stopped them all moving together.

I had assumed that the MS2 would have automatically initiated contact with all the mfx locos and that there wouldn't have been any need to further link them together by sending a "driving" command - but apparently not.

I moved the Brawa loco from DCC address 3 to 16 and this has completely cured the problem.

Lesson learned - move any DCC locos off the default address of 3 as soon as you get them because Marklin mfx locos still listen on this address?

Thanks again for the help.

Hello. If your locos Support Mfx and DCC, and mfx and DCC are enabled in the MS2, your mfx locos will not respond to DCC sent command, even if they are DCC capable.
Regards

Märklin recommended anyway that you must disable unecessary protocols in the locomotives decoder.
You do this at CV 50 by follow the locomotives manual.
This to avoid digital interference.
I use only one protocol in the digital system too.

No Brawa use D&H sound decoder and use DCC and MM protocols.

I own one DCC Loco, several MFX, and MM Locs. so I need all protocols active. I've not encountered an issue yet.

yes Goofy, it says that, and also that DCC and MFX can confuse each other.

Theoretically this should not happen. mfx was designed with DCC in mind.

But the model of having different priorities for different protocols does not really work flawlessly and sometimes decoders get confused by attending to two protocols at the same time when they should obey a single protocol only.

But you're still wrong... the manual refers to the settings in the CS3, NOT that you change the CVs of the locomotives.

The manual (pdf streaming link: https://streaming.maerkl..._Manual_EN_final-lo.pdf) says on page 41:
Important: The system with the most possibilities is the highest ranked digital protocol. The sequence of digital protocols is in declining value:
Priority 1: mfx – Priority 2: DCC – Priority 3: MM
When more digital protocols are recognized, the decoder selects the highest ranked protocol.
Note: Protocols that are not needed should be deactivated to avoid malfunctions (see page 23).

Page 23:
Shows a picture of the track settings window on the CS3 with tick boxes (on or off) for mfx, mm2 and dcc and the text in the manual says: Here, you can turn off protocols not being used (see page 41).

There is no indication of recommending the individual locomotive decoders be set to only one protocol. I know some people only use dcc and therefore only use their mfx locomotives in dcc mode - because they want total control of various CV parameters. But Märklin doesn't recommend only using ONE protocol.