Sometimes after I grant a title to a landless councilor, he becomes ineligible for the seat for a while: why?

I've noticed several times that if I have a landless council member and I grant him a landed title, his seat becomes vacant and I cannot appoint him again.

After an unspecified amount of time he becomes eligible again.

Why is that? Is that a bug or is there a reason for that?

I have seen this also. Maybe it's a bug, but if it is it's a fairly minor and predictable one. From what I have seen every time I have ever landed a Courtier Councilor he has been removed from the Council. If I wait a few days I can go back and re-appoint him (or sometimes appoint someone else and get to dodge the 'fired from council' opinion hit). I think I understand why it happens.

All of your councilors are eligible for one of two reasons. Either they are one of your Courtiers, or they are one of your Vassals. When you land a member of your Court he stops being one of your Courtiers and I believe this triggers the game to mark him as ineligible to be on the council and removes him. Of course, he then becomes your Vassal and is eligible once more, but from the game's perspective, the reason he was eligible briefly stopped existing. I suspect that little hiccup when the councilor eligibility changes causes him to be removed.