An odd problem with Bachmann decoders

[thenudehamster]

I have three Bachmann locos which are factory equipped with decoders - an Alco RS3, Alco 2-6-0, and a GE 44-tonner. The RS3 is no trouble (albeit slow compared with a similar Atlas greyhound!) in that I can reprogram any CV quite happily. The 44-tonner and the 2-6-0 exhibit a peculiar error in that I cannot set CV29 to any number which uses the 28-step speed table (CV67-94). Both decoders will accept the speed table, but won't acknowledge the CV29 values to use it!
Programming using JMRI gets an error message that it receives no Confirmation from CV29; using my MERG CANCMD and CANCAB shows CV29 to program OK, but re-reading, the value shows no change. It seems, therefore, to be a decoder issue.
The literature assures me that the decoder (the exact model is not stated) in both locos can use 28-step speed control, so I am a little puzzled as to why they will not accept 28-step programming.

Any ideas?

Barry