USB-C power output from M1 Pro MBP is much lower than older MacBooks

I often watch Netflix on my iPad while working on my MacBook Pro, and when the iPad battery needs recharging, I plug it into one of the MBP's USB-C ports.

On my old 16" MBP 2018, the iPad will charge up while streaming a movie.

On my new 16" M1 Pro MBP 2021, the iPad will not charge up. Either the iPad shows the charge icon, but the battery % remains the same over a long period (or it drops slowly); or worse after a while it no longer shows the charge icon at all, despite being plugged in and nothing has changed on either end.

Looking at About this Mac > System Report > USB, I can see the iPad showing up in the USB Device Tree.

On the MBP 2018, here are the power figures:

Current Available (mA): 500

Current Required (mA): 500

Extra Operating Current (mA): 1900

Sleep Current (mA): 2400

On the M1 Pro MBP 2021, I see these lower figures:

Current Available (mA): 500

Current Required (mA): 500

Extra Operating Current (mA): 1000

Sleep Current (mA): 1500

I tested this in an Apple Store with another M1 MBP, and got the same low figures.

I've tested with 3 qualities of cable (thin USB-C cable that came with the iPad; thicker MBP charging cable; and a thunderbolt cable) - all show the same low figures.

The only reference/documentation I can find is this thread https://discussions.apple.com/thread/250595853 - it links to an AppleInsider article from 2016 which suggests a MBP should be able to output 2100mA over USB-C (and their screenshot of the USB devices screen shows 500, 500, 1600, 2100mA). My output from the 2018 MBP is even higher at 500, 500, 1900, 2400mA), but the M1 Pro MBP is much lower.

It's not clear if this is a downgrade in hardware, or a software limitation/bug.

Either way, it's incredibly frustrating, as I can no longer keep devices charged via my laptop.