I'd like to downgrade that to a solid Maybe. 🙂

There's no such thing as joint custody in the IRC.  It comes down to counting nights.  The "custodial parent" is the one who wins the night count.  See page 4 of Pub 503 (2021):

https://www.irs.gov/pub/irs-pdf/p503.pdf

"The custodial parent is the parent with whom the child
lived for the greater number of nights in 2021. If the child
was with each parent for an equal number of nights, the
custodial parent is the parent with the higher adjusted
gross income. For details and an exception for a parent
who works at night, see Pub. 501.

The noncustodial parent can't treat the child as a quali-
fying person even if that parent is entitled to claim the
child as a dependent under the special rules for a child of
divorced or separated parents."

Rick