Electronic – Isolated CAN transceivers as a CAN bus extender

I am designing a system to 'hack' into a CAN bus. My first idea was to split the target CAN bus, terminating both ends into isolated CAN transceivers, and use a MCU to pass packets between the buses while altering values inside the packets. Below is a rough picture of the setup:

My problem is that while connected to the target CAN bus, the system fails to transmit on either CAN bus. In the program I can see it is receiving, but the MCU gets stuck in a loop of failed transmissions.

The weird part is that I can prove the code and hardware are all viable in a different environment. When I hook up the hardware to a test rig I made using a couple of development boards and a power supply, it works well! The packets from both devices are seen on both buses. The packet bits look healthy and square. I am at a loss figuring out what specifically is different about the environments on my desktop and on the target CAN bus that make it break.

System details follow:

MCU – STM32F746ZGT6

Isolated CAN transceivers – ADM3054

Bus speed – 125kHz (confirmed with oscilloscope and CAN sniffer tool)

I'd really appreciate help thinking through this system design, and understanding how it might cause CAN transmissions might fail.

Thanks!

EDIT:
Upon farther thought, I think I realize the area to focus on is between the TxR and the MCU. If the MCU is refusing to declare a successful transmission, than it must think something wrong is happening on the bus. It can only interact with the bus via TX and RX. I will scope TX and RX to see if I can see any weird errors, possibly caused by reflections or something interfering with RX during transmission?

EDIT 2: added schematic and layout image