Works fine, except for one ‘small’ issue. Post loses pressure if left down for more than a few seconds at a time. To the point that I cycled it a few times after last bike wash and could hear air leaking out when the post was down.

If the post is left up holds pressure absolutely fine.

Has recently had all new seals and bushings.

Is it terminal or fixable?

Should be a seal change. If I had to guess I’d say one of the ones inside the inner seal head. So the grey bit inside the green but in the middle as shown here. I think that’s the only one that should behave differently depending on post position. It could also be that the increase in pressure is enough to cause a leak somewhere. If it’s not interfering with the lock then it shouldn’t be something on the IFP.

That’s not for certain though, I’m not having much success finding the air leak in mine and I’ve replaced most of the seals…

If you’ve not serviced it before and it’s still in warranty I’d see if you can get it changed. If it’s not in warranty if probably still look at getting someone to do it for you, it can be a major pain.