A person performing ‘Umrah is traveling by plane and has been informed that he will pass above the Meeqat in thirty minutes, but he made the intention and started the Talbiya shortly before the Meeqat – is that considered a problem?

There is no problem with that caution and vigilance due to the speed of the plane
as long as he did it shortly before the Meeqat, as the requirement is to make the
intention while being at the Meeqat or aligning to it.