26 month old suddenly waking between 1-4am every night since turning 2. Any advice?

My 26 month old has been sleep trained since she was 6 months old using CIO. She’s generally always been a good sleeper apart from the usual disruptions like illness, teething etc.

Since she turned 2 though, it’s honestly like a switch has been pressed. She’s now waking anywhere between 1am and 4am pretty much every single night.

Her schedule used to be:
Bed: 7:30pm
Wake: around 5-6am
Nap: 12:30-2pm

We thought it might be a schedule issue so we’ve tried changing things around. We moved bedtime to 8pm and also moved her nap to 1-2pm and capped it at an hour. It hasn’t made any difference.

Her room is temperature controlled, she’s not ill and it’s completely pitch black. We’ve blacked out the windows and covered any little sources of light, so I don’t think it’s environmental.

When she wakes she cries for mum/dad and milk. If we don’t give her milk she just keeps going. We’ve tried CIO again multiple times and the two longest attempts were around 90 minutes. She’ll cry, go quiet for a bit, then start crying again. She just doesn’t seem to go back to sleep.

Eventually if we give her milk she’s usually wide awake and ready for the day. If we bring her into our bed instead, she’ll literally spend 2-3 hours wriggling around, mumbling, playing etc before she finally gets tired enough that one of us can pick her up and walk with her for 10 minutes and she’ll fall asleep.
The milk is probably the hardest part. If we don’t give it to her she goes absolutely insane and nothing else seems to settle her. We’ve tried comforting her without milk and we’ve tried just waiting it out, but she’ll keep crying and asking for it.

It really feels like some sort of split night but I can’t work out what’s causing it. We’ve tried changing nap length, changing nap time, later bedtime, CIO again and making sure there’s nothing environmental waking her.

Has anyone gone through something similar around 2 years old with a child who was previously sleep trained?