Thanks for the example montage.
The problem was that the montages were selected based on the channels available in the figure instead of all the channels available in the channel file. If you were displaying MEG signals, your montage was not selected because it does not contain any MEG channel. But it was available when reviewing EMG signals (right-click on the data file > EMG > Display time series).
As this was not very intuitive, I changed the logic to allow all montages matching any channel of the channel file, even if this is not the appropriate modality that is displayed in the first place:
https://github.com/brainstorm-tools/brainstorm3/commit/56e2c299b4b0a1f05bc416da505cb0d145b41530
Update Brainstorm to get this fix.
However, this would also have other undesirable side effects. If you display the EMG+C4 channels starting from an MEG time series figure, you would get all the contextual menus and default options associated with MEG signals, because the figure is still registered as "MEG".
Therefore, for displaying montages that are mostly containing EEG or EMG channels, prefer using the appropriate modality in the first place (right-click on data file > EEG/EMG > Display time series) instead of simply double-clicking on the file (with opens the MEG by default).