The Mighty Servant of Leuk-O is a mech that creatures can pilot. Its description says, "Those inside the servant have total cover from effects originating outside it."
It does not say that it grants total cover for creatures outside the servant from effects or spells originating inside the servant. I want to know if this is RAI. It seems like pretty specific wording. If they had added the words, "and vice versa" to the end of that sentence it would be explicitly two-directional.
Are there any other instances where the rules create asymmetrical total cover?
The description continues, "The controls within it allow creatures to see outside without obstruction." It does not clarify how exactly this happens. There could be some magical equivalent of flatscreens inside the servant, or it could be a one way force field that allow the pilots to see out the eyes. The description is ambiguous on this count.
A glass window provides total cover per this sage advice, but the description of Leuk-o does not actually mention glass or anything transparent.
Most spells that create something that grant total cover don't explicitly mention the cover at all. For example, Wall of Stone does not have the word cover in its description.
Best Answer
Based on historical evidence and total cover descriptions in the rules, the servant is unlikely to grant one-sided total cover. But ask your DM
There are magical effects that provide a near equivalent of one sided total cover, like Leomund's Tiny Hut (it's not exactly the same as you can not cast spells out of it).
Other spells do not describe the obvious, because the game expects you to apply common sense: if an object like a wall blocks line of effect, it grants cover to both sides. This can be asymmetrical for partial cover, but not logically for total cover which blocks all possible lines.
So the question is if the Servant has such a magical one-way force field that allows you to look out, or if it has physical windows, which would provide total cover to both outside and inside.
If we are allowed to take guidance from earlier editions, we can look at the description in 1e:
Here the mention of crystal suggests that the way to look out are crystal segments. This is supported by the depiction of the sister item, the Machine of Lum the Mad: here the internal chamber is represented as a crystal compartment.
Another, similar item is the Apparatus of Kwalish. Here, it is explicitly stated that it has window panes on the front and sides (that can be shuttered).
The Siege Tower (DMG p 256) uses the same language. Historically such towers were also used to gain higher ground than the defending walks and clear them. The tower in the DMG instead is described as just a safe way to get up:
Lastly, the Ram, also under Siege Equipment contains this line:
Nothing is mentioned about those outside the tower or above the ram also having total cover, as this is self-explanatory, at least for the ram. So there is precedent in the rules for describing two-way total cover by just stating one side has it.
One sided cover is sufficiently rare in the rules that one would be justified to expect something is explicitly stated about it. It often is difficult for the writers to spell out things that seem self-evident, so the absence of a clause regarding cover both ways may not necessarily mean there is one way-cover. I think, as far as one can sleuth about RAI, there isn‘t.
However, as you say, the description of the Servant in 5e does not really clarify how creatures can look out, and even the crystal might have magical properties to allow one-sided effects.
Therefore, your DM will have to decide how they rule this to work, like in many corner cases that do not have a clear-cut answer one way or the other.
That is also covered in Tasha‘s Cauldron of Everything, on page 4: