Jace, the Perfected Mind

• WOTC Official Reminder Textofficial rules
  Compleated — Phyrexian mana can be paid with 2 life. If life was paid, this planeswalker enters with 2 fewer loyalty counters.

• WWizards of the Coast2023-02-04
  Official Ruling
  Other replacement effects that would change the number of loyalty counters Jace enters with will apply as normal.
• WWizards of the Coast2023-02-04
  Official Ruling
  Jace's second loyalty ability doesn't care which player's graveyard has twenty or more cards in it. It could be the target player's or another player's.
• WWizards of the Coast2023-02-04
  Official Ruling
  A Phyrexian mana symbol contributes 1 toward the mana value of a card, even if life is paid for it. Specifically, Jace's mana value is always 4.
• WWizards of the Coast2023-02-04
  Official Ruling
  The compleated ability looks only at whether a player chose to pay 2 life for a Phyrexian mana symbol as they were casting the spell. If a player paid life for some other reason while casting the spell, that will not reduce the number of loyalty counters the planeswalker enters the battlefield with.

Rulings via Scryfall