I've created a (sort of) minimal example of the issue. I'm attaching both the TeX source and the resulting standalone document converted to PNG.
I hope the issue here is self-explanatory: I'm creating a group plot and configuring it such that that plots in it should have certain labels with corresponding units, although not all of them will be displayed later (because of y descriptions at = edge left). This works quite well but it appears to affect axis environments used for colorbars, too. This is surprising because only the corresponding units appear to carry over, not the labels. I've commented out the code colorbar style = {use units = false} which serves as a workaround, but I personally view this as a bug.
Please find the aforementioned PNG attached.