The problem in your case is linked with the track numbering.
Each "TRACK (nr.) AUDIO" line within a .CUE file defines a track with track number (nr.) for the concerned album.
Thus, as you have defined your two .CUE files, you end up having:
- Several tracks with nr. 1:
- Father's Shout (1a in your picture)
- Rise and Sunshine (5a in your picture) - Several tracks with nr. 2:
- Breast Milky
- If
- Sunny Side Up - Several tracks with nr. 3:
- Mother Fore
- Summer '68
- Morning Glory - etc.
To solve this, you must keep the track numbers in sequence throughout the whole CD and the different .CUE sheets.
In this case:
- The .CUE sheet for "Atom Heart Mother" (originally track 1) will contain tracks 01 -> 07
- "If" becomes track 8
- "Summer '68" becomes track 9
- "Fat Old Sun" becomes track 10
- The .CUE sheet for "Alan's Psychedelic Breakfast" will contain tracks 11 -> 13.
N.B.
Just to be sure: a .CUE sheet must include the filename at the beginning,
but you have probably not included that part in your post.
(See Wikipedia in EN and IT, or more details on HydrogenAudio (EN)).
How did you manage to display the subtrack IDs (1a -> 1f, and 5a -> 5c),
since REM clauses are ignored by MusicBee?
For the sake of legibility, it is best to indent the .CUE sheet lines (as in the Wikipedia examples), but it is not mandatory.
I seem to have already tried to do as you suggest, but I have made so many attempts that I am not sure. I will try it and then report the results.
On reflection, if track two becomes eight, the importance of numbering the tracks with 1a.1b,1c... is no longer relevant.
I wanted to do the discoGS style, replacing the numbering (in this thing is vinyl) with the more convenient CD numbering.

The REM command works, you have to remember to put the desired field completely blank in all tracks taken into account, before updating from the cue.
It seems that if it finds the field with any data it will not insert the data from the REM.
Then I created a virtual field with the condition if the field tracciaST is not empty put the data it contains, otherwise put the track number.
This does not work either. The only doubt is the index of tracks 8-9-10-11 (2-3-4-5) which I set to zero because they are separate files from the first one. I don't think track 8 (2) should be told to start at 23 minutes and 44 seconds.
PERFORMER "Pink Floyd"
TITLE "Atom Heat Mother"
TRACK 01 AUDIO
TITLE "Father's Shout"
PERFORMER "Pink FLoyd"
REM tracciaST "1a"
INDEX 01 00:00:00
TRACK 02 AUDIO
TITLE "Breast Milky"
PERFORMER "Pink FLoyd"
REM tracciaST "1b"
INDEX 01 02:50:00
TRACK 03 AUDIO
TITLE "Mother Fore"
PERFORMER "Pink FLoyd"
REM tracciaST "1c"
INDEX 01 05:23:00
TRACK 04 AUDIO
TITLE "Funky Dung"
PERFORMER "Pink FLoyd"
REM tracciaST "1d"
INDEX 01 10:13:00
TRACK 05 AUDIO
TITLE "Mind Your Throats Please (Part 1)"
PERFORMER "Pink FLoyd"
REM tracciaST "1e"
INDEX 01 15:28:00
TRACK 06 AUDIO
TITLE "Mind Your Throats Please (Part 2)"
PERFORMER "Pink FLoyd"
REM tracciaST "1f"
INDEX 01 17:42:00
TRACK 07 AUDIO
TITLE "Remergence"
PERFORMER "Pink FLoyd"
REM tracciaST "1g"
INDEX 01 19:12:00
TRACK 08 AUDIO
TITLE "If"
PERFORMER "Pink FLoyd"
INDEX 01 00:00:00
TRACK 09 AUDIO
TITLE "Summer '68"
PERFORMER "Pink FLoyd"
INDEX 01 00:00:00
TRACK 10 AUDIO
TITLE "Fat Old Sun"
PERFORMER "Pink FLoyd"
INDEX 01 00:00:00
TRACK 11 AUDIO
TITLE "Rise and Shine"
REM tracciaST "5a"
PERFORMER "Pink FLoyd"
INDEX 01 00:00:00
TRACK 12 AUDIO
TITLE "Sunny Side Up"
REM tracciaST "5b"
PERFORMER "Pink FLoyd"
INDEX 01 03:33:00
TRACK 13 AUDIO
TITLE "Morning Glory"
REM tracciaST "5c"
PERFORMER "Pink FLoyd"
INDEX 01 07:45:00