I am unsure if it is generally appreciated that many of the Bartok functions that are usually controlled via Mosaic (wifi) will also respond to IR commands such as from the dCS Universal Remote.
The obvious answer would be that some of the codes that ARC have programmed are identical to those used by dCS for the IR control of functions on the Bartok. Both may use codes based on the Philips RC-5 protocol. The duplication should not happen but I remember at least one command to my old Panasonic TV from its remote would operate one of the functions on my Paganini stack. However as I would imagine that the combination of ARC amplification and dCS digital processing is pretty common around the world ( I was actually using one a few weeks ago with no issues) I think that although duplicate codes are a possibility they are unlikely.
However what deepens the question is that the ARC LS26 remote is not controlling the LS26 itself !
Are you sure that the remote will not control the LS26 because the LS26 is itself not functioning correctly? Can you get all of the LS26 functions to work by manual selection from the front panel? NB: I cannot see any description in the LS26 manual that indicates that there are circumstances whereby the remote will not function or that it needs to be activated in some way.
Are you sure that you have not sited the preamp in such a way that that the window for the IR receiver is being obscured? BTW I am guessing that it is placed within the display panel on the fascia. In some pieces with IR windows placed there the angle of acceptance for the IR signal is relatively narrow as the window is actually positioned deeper than the front surface of the fascia and can then only accept the signal over a narrow area. Have you tried the IR control from various positions including directly in front of the LS26 and not too far away? I expect that these simple points may provide a fix. If not then we need to consider things such as the remote, whilst being from ARC, is not the correct one for this model.