Arduino system ups problem!
First, TP4056s are limited in their current draw by a resistor on the PROG pin.

What is this resistor value on your UPS?

Second, where are you measuring current from the truck battery, on the 24V side? P=VI so at higher voltage current will be less, 150mA at 24V is approx 720mA at 5V. That would be enought to supply the Arduino system and charge the battery.
If this doesent solve your problem then, you have already started breaking down your problem, I would encourage you to continue the process.

Can the car battery provide enough power? The battery will likely have no problem with this but as you mention it is disconnected when the car is off there is some circuitry inbetween that may limit this.

While the car is on can it power the Arduino system with the selected buck converter? This is a combination of Q1 and can the buck converter provide enough power at the supplied voltage.

Looks like you are using an external battery pack in place of your 18650. Is there any circuitry there that could be interfering with the logic on the UPS?

What point in the battery charging cycle is it? If the battery is at full capacity or close to it then the UPS may accept less current from the buck converter and draw from the battery as well. Depending on design.

Is there power path management (or similar)? Meaning does all the power for the Arduino system come from the battery and it is charged at the same time or does the UPS allow for power to flow &quot;around&quot; the battery, directly from the buck converter to the Arduino system? Depends on UPS design.

The UPS includes a FP6298 boost converter, are there limits there similar to the TP4056?