If you connect the signal from the measuring instrument directly to the resense, it will oscillate.

How many centimeters is the cable extended?
If you pull the signal directly connected to the input pin of the CMOS-IC out of the board and use it, you can only extend it by about 10cm.If you pull it out more than that, it will become unstable as pointed out (experienceally, it will always fail if 15cm is close to 20cm or more).It's only one of those cases where noise gets on and oscillates.

Whether it's USB or SATA, or RS232 (EIA574) IEEE 1394, it's not Date that has a dedicated circuit on the serial communication bus.

Stable signal reception without inverter

As long as the CMOS signal output equipment 1 and CMOS signal input equipment 2 are directly connected to each other over 10cm, it will never be stable.

• Convert to a signal suitable for medium and long distance transmission (RS232 or RS485 or LVDS)
• Connect at the shortest distance and allow the total extension to be less than or equal to one digit cm
• Insert C/R/D for noise protection

It's a countermeasure that can be taken.Specifically, the circuit you choose depends on the usage and cost, such as distance, speed, and ambient noise levels.

C/R/D alone has a reasonable effect (if it's a short distance), would you like to try it?If you want to know the specific circuit, search it (I can answer it with Oira, but I don't think it's a topic for SO)

This is what the official website says.

https://developer.sony.com/develop/spresense/docs/hw_docs_ja.html#_ Digital Signal uartspipwmgpio on Expansion Boards

If the impedance of the input signal is low.It looks like you need a buffer.
If so, why don't you put in a buffer?