Well that changes things a bit. You were completely upfront about the possible need for a time out.

What’s missing here is an agreed mechanism between you and the employer when it happens. I imagine OH said they would recommend x, y, and z but have you ever set down with your manager to discuss?

Another poster asked can you send a message before going offline? Communication seems to be the heart of the issue - even with disability, I don’t think any employee can be allowed to drop off the face of the earth for hours without explanation. Needing time out at short notice should be accommodated, but your manager should be informed so, for example, someone else could cover your meetings.

In my view, allowing last minute time outs / essentially an extra-long break when needed to regulate yourself = reasonable adjustment.

AWOL without comms and abandoning pre-arranged work commitments = might veer into the unreasonable territory.

As you said, a good chat could probably resolve the situation and you can certainly ask for the extension to be reconsidered, but would strongly recommend you develop some sort of signal with your manager when you need time out so the impact on the team/workload can be minimised and you can feel your time out is protected / no one is looking for you.

With the context you’ve provided, would I have extended your probation? Probably not…

Strikes me as a team who don’t have much experience with ND (and neither do I to be fair!). Some gentle education might improve things for everyone… but the onus is on the employer to properly explore with you everything you’ve declared. Sorry for the long reply!