Funnyplaying GBA custom upgraded motherboard replacement booting in GBC mode without cartridge

[whole valley]

Hello,

I've been transplanting GBA CPU and RAM to a Funnyplaying GBA custom upgraded motherboard replacement PCB (https://funnyplaying.com/products/gba-custom-upgraded-motherboard-replacement) and now it boots to GBC mode with nothing inserted. IN35/Pin 2 on the CPU is 3.5V and I'm at a bit of a loss what to try next. When I'm measuring Voltage between GND and C2 the console weirdly boots into GBA mode.

Is it possible the switch on the Cartridge slot is damaged? If so, how would I troubleshoot that?

Any help appreciated, thanks in advance.

[solemn ridge]

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[whole valley]

It does boot GBC games, but GBA games also boot GBC mode and nothing happens. If I stay with the multimeter between GND and C2 it boots into GBA mode and actually starts the game.

[worthy ocean]

What's the resistance across this resistor?

[whole valley]

In-Circuit it's unstable, starts at ~8.3 kOhm and falling.

It... broke into two pieces when I unsoldered it. 🤔 Do you know what it's supposed to be?

[worthy ocean]

0603 100K. IN35 should be pulled low when in GBA mode so I'd try replacing that resistor.