With my limited understanding I see the following:

The FDCAN implementation on the STM32G0B1 uses interrupt "TIM16_FDCAN_IT0_IRQn", which calls "TIM16_IRQHandler(void)" (in HardwareTimer.cpp).
In "timer.h" it says: #define TIM16_IRQHandler TIM16_FDCAN_IT0_IRQHandler.

"TIM16_IRQHandler" should analyze the interrupt source and forward to "HAL_FDCAN_IRQHandler" if the FDCAN bus is the interrupt source so the FDCAN callbacks can be used/called.

Currently TIM16_IRQHandler only processes interrupts triggered by Tim16, so the FDCAN bus callbacks cannot be used, and even worse, the interrupt flag is not cleared causing an infinite loop.

"TIM16_IRQHandler" in HardwareTimer.cpp

void TIM16_IRQHandler(void)
  {
    if (HardwareTimer_Handle[TIMER16_INDEX]) {
      HAL_TIM_IRQHandler(&HardwareTimer_Handle[TIMER16_INDEX]->handle);
    }
  }