Can Portions of States Secede from Those States?

This is in the news again, in San Bernardino County—a county next to L.A., which is home to over 2 million people (AP):

The county's Board of Supervisors voted 4-0 on Wednesday to put the secession measure on the 2022 ballot, the Southern California News Group reported. One supervisor was absent.

The measure will go before the board again next Tuesday for final adoption. The initial draft would put this question to voters on Nov. 8: "Do the citizens of San Bernardino County want the San Bernardino County Board of Supervisors to study all options to obtain its fair share of state and federal resources, up to and including secession?"

Can they do that? Why, yes, they can, but only with permission of the state from which they're seceding as well as of the federal government. Article IV, section 3 of the U.S. Constitution provides,

[N]o new State shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.

The provision allowing such actions with the states' and federal government's consent appears to apply to both the "erected within" clause and the "formed by the Junction" clause, and indeed this is what happened when Kentucky seceded from Virginia in 1792 and Maine seceded from Massachusetts in 1820. (I set aside the West Virginia / Virginia precedent, which was complicated by the Civil War.)

Our own Eugene Kontorovich has argued that when part of State A seeks to join State B, the consent of State A might not be required, since that's neither erecting a new state within state A nor forming a new state. But I'm inclined to think state A's consent would be necessary: If San Bernardino County joins Nevada, that is in effect forming a State (albeit one that would also be called Nevada) by the Junction of two "States, or Parts of States" (the State of Nevada and Part of the State of California).

So the Constitution forbids nonconsensual secession from a state (secession without the consent of the state that loses territory); but it doesn't forbid consensual secession.