TMS320F28P650DK: Disable external channel selection for specific SOCs

Part Number: TMS320F28P650DK

Tool/software:

Hi!

I'm using ADCA to sample four high priority inputs (SOC0 to 3) at switching frequency. SOC5 to 10 are sampled at the same rate with round robin priority.
Further, an external 3-bit multiplexer with four temperature sensors is mapped to SOC12 to SOC15, which is triggered at a rate of 1 kHz. SOC12 to 15 are in the round robin wheel as well.
I want to use external channel selection for controlling the mux. Due to high input capacitance, preselection of external channels is required (EXTMUXPRESELECTEN = 1).
Unfortunately, SOCs other than the external mux SOCs output the default value EXTCHSEL = 0 and interrupt the precharging process of the pending temperature channel.
This results in corrupted temperature values for all channels with EXTCHSEL not being zero.
Is there a way to disable the external channel selection for SOCs that are not related to the external mux?

Thanks!