Thanks for the advice. It was on the lowest level of the schematics.
I had used a GND symbol with the value “GND_${SHEETNAME}” to get a local power plane but it den was still connected across the copies of the 8ch_cell_emulator because the 1ch_cell_emulator sheets are shared. Using just a local net label solved the issue.
This thread was also helpful (but gave at first the “GND_${SHEETNAME}”, without mentioning the caveat): Local power on a Hierarchical sheet - #15 by paulvdh