Lower quality filament can drift in colour on the spool. When printing one object, it’s not really noticeable as a small amount is used. When printing a plate filled with objects more filament is used per layer and you will see the variation.
One option is to print by object as that prints each part at a time. The downside is that you can’t place as many on the plate.
To me, it seems like the printer is slightly losing its position. A couple of things could be causing it:
Loose belts
Loose printhead
Calibration needed
Does it also happen when you do single-color prints?
If it only happens during multicolor prints, maybe when the printhead moves to the prime tower or the filament-change area, something is causing it to shift slightly and lose its position.
I have the same issue and detected, after a lot of troubleshooting via Bambu Studio setting changes, play in the print head bearings. I contacted Bambu Support and they decided to change the print head carriage. Since the printer was 4 weeks young I requested a replacement printer, which I have received yesterday. The first thing I have done (before any print) was checking the print head for play. Unfortunately, it shows exact the same play. I have measured a displacement of the nozzle tip of 0,12 mm by using a dial gauge. From my point of view this is enough for these layer shifts. As expected, the prints show also the same uneven walls (as it can be seen on your picture). I have raised a new ticket a few hours ago.
I read about play of the toothed on the X axis somewhere here before and I wonder if this really is a fault. Looking at this guide below you can see that the toolhead mounting has springs pressing against the aluminum sleeves of the axis, probably to address such play between the sleeves and the toolhead.
I can feel some play too but I have to use quite some force to do so hence I suspect that I‘m working against those springs.
Is it easy for you to move the toolhead to feel its play or do you have to use some force, too?
Yes, I need a slight force for that, but due to the high accelerations and weight of the print head, this is easily reached during the print. The thing is, that the displaced nozzle does not „spring back“ to its initial position when removing the force. Moreover, the Bambu Support has also decided that this play is not normal, since they proposed a carriage replacement. Nevertheless, I see a changing gap between bearing and rod, so the play is coming from the bearings, not the carriage. When applying the same force to the bearings of the Y-axis, I do not notice any play at the bearings or a nozzle movement on the print head. My previous printer Prusa MK3S has also not such a play and printed flawlessly until the last day.
Here my measuring method, unfortunately I cannot upload a video.
Not sure about loose belts, maybe I should recalibrate the belt tension. I have done full calibration, and it is still the same, have not done single color print of this many object, thought I supposed I should try in near future.
Thanks for the write up! I‘m interested to hear what the support comes back with. I also have slight play but I don‘t specifically have issues with uneven walls. Only occasionally at places where layer times are different where I might get layer shifts. Still I‘d like to hear how Bambu Lab sees this. If you don’t mind keep us updated, please! 
I have already received an answer from the support, but this was quite useless: I should contact my reseller for further support. I don‘t know what he should do to solve this.
