Please check where you are deriving your signal for the split from.
If you are sending "Input Direct Output" signals, no matter where you set the Direct Out source to be (MIXRACK, Mixer Pref, Preferences, Global Direct Output Source etc.), the Trim control will vary the level of the send.
This is because the Trim control is logically located in the PreAmp, and the earliest in the channel signal path that you can select for the Direct Out source is post-PreAmp.
To get around this, you need to change the source of the signals that you're sending by pressing the OUTPUTS button under the touchscreen, then selecting the Port B tab.
From here, you can change the sources of one or more channels at a time.
For the channels in question, change the source from Input Direct Output, to MixRack Inputs.
This is the only source that will send a signal before the Trim control.
Any other setting will mean that either the Gain or the Trim controls will affect the level of the signal sent to the downstream connection.