I've played with this a bit in my current rig in manual mode; it's not an IG of course so it depends on how they've set it up, but it does what you describe - protecting the bits and bobs when the manual ask is too great for what the computer decides is OK - so I have no doubt the IG has a similar system. The question is how it does it operate? What I've noticed on mine is the following:
1) If speed is high and one tries to force a downshift, it will only allow you to do this within a certain speed, depending on the gear. For instance, if I'm doing 60 KPH, it won't let me shift down below 3rd gear, which is good - doing so would cause the engine to rev way too high and cause damage. It will abort the shift and give me a dash message saying "shift denied, speed too high".
2) If speed is too low, it seems to treat the selected gear as the highest possible gear, but then will shift normally between the lower gears all the way up to that gear. So if I manually select 4th gear, it will shift through 1 through 4 as normal, but then won't go higher than 4, and the limiter will of course limit my speed as well. I actually don't like that - it's not really clear to me what gear I'm in at a given time, and there are situations where I want it to "hold" the gear, not shift through lower gears but not exceed a max.
I'd much prefer a simple system where the ECU has Green, Yellow and Red parameters - Green is when your RPM/Speed/Manual Gear selection is a-OK. Yellow is when you are on the outside edge of OK, and may need to pay attention to speed or RPM to ensure you stay within spec; Red is "we are gonna shift for you now because you're gonna break it". Correspond these to simple and subtle indicators on the dash if I need to know what the vehicle is thinking of the gear I chose, and I'd be thrilled with a car that let's me mostly lock it into gear as I need to.
Long story short though - losing power going up hill sounds like an unusual way to manage an automatic transmission in 2023, but it depends on how Ineos has it programmed to behave - does anyone else have time to go fiddle with the manual gear select and let us know how that works?
Really curious what the cause of this power loss might have been!