Unable to capture artefacts since area is unreachable (Arcane Prison)
See screenshot below.
See screenshot below.
Interesting! You should change the campaign to include this in some way to teach the player this feature.
It is mentioned in campaign level 4 where you need to beat underlord Marcus, but it is not required to complete the level.
I think many people just ignore such hints when they are not forced to use it.
As mentioned by v0id this is working as intended (although you're not the first person to assume something is wrong, it's easily done) so it isn't a bug.
Marking as Declined. (I miss the "Not a Bug" option)
This is not a bug, you need to place an Outpost construct there to get this artefact.
Simply use prophecy and then place the construct as long the area is revealed.
It allows to claim areas not connected to your remaining claim.