Yes. An oddity. I don’t see how it amounts to very much, though.
If there is no liturgy being celebrated, for a few brief hours, I don’t see any problem with saying that Easter Time ends with Vespers II of Pentecost while Ordinary Time begins on Monday.
Let’s keep in mind that adding the alleluias is something that “is done” during Easter time. The phrasing is not “do not add during O.T.” (in the way that we are told omit/substitute the Alleluia during Lent at Mass)
Until the calendar revision, Easter Time ended sometime on Ascension Day, AND there was a Pentecost season.
I can easily see that when the revision was made, the ideas were put-forth (and approved, etc.) that a) Easter would be extended until Vespers2 on Pentecost and b) Ordinary Time would resume on Monday, maybe (just maybe) the question of “what season applies to that few hours in the middle?” did not present itself.
So, if I pray Vespers 2 of Pentecost at 5 PM on Sunday, then pray Compline at 1:00 AM on early Monday, does it really matter what season-name I apply to the intervening hours?
Now, if a parish has a late Mass on Sunday (maybe 7 PM, which would be after I pray Vespers in private), there’s no doubt I would use the Pentecost Mass. When there is Liturgy, defining the season is surely important.