Yeah, the settlement period is definitely a thing, applies to professional traders as well as retail.
The clearing house (DTCC is probably the largest one) takes possession of the shares you sold and the cash the buyer paid, “sits” on this for two days* and then the cash proceeds arrive in your account (and the shares in the buyer’s account). As an aside, everyone took a little cut along the way, via spreads, transaction fees, you name it. Gotta love these market makers …
So, at any rate, two days later is when you really have the cash and the buyer really has the shares.**
Depending on the risk management of your broker and how much they trade on their own book (shares/cash exchanged between you and positions they, your bank/broker, holds on accounts they possess, you might get access to your cash/shares sooner than T+2.
But if the buyer of your shares is in Tokyo with his Japanese broker, and you sold them from Switzerland with IBKR UK, then things will definitely be settled via a 3rd party, and the value date of your transaction will be two days after your trade date.
The settlement period of T+2 of course also applies to FX transactions. A so-called spot transaction (sell Yen buy USD at the current / spot exchange rate) also settles only two days later.
You get the impression that currencies can be exchanged immediately by taking cash in Yen to the bank and exchanging it for USD cash, and indeed you get your USD cash immediately, but the spread is much wider for cash exchange than for electronic transactions (well, unless you’re at probably any old Swiss bank, where the spreads are probably about the same for cash and electronic currency exchanges, if you squint a bit … Anyhow, I’m getting a little off track).
When you do the currency trade at a bank/broker at the spot rate, T+2 applies as well. I.e., if your balance in USD is zero and you sell Yen for USD today, the USD won’t be avaiable to you until two business days later.
In your case I would guess that previous trades in USD only might have been on the book of IBKR with no currency exchange risk involved, while your recent trade with selling stock in Yen and spot buying USD includes FX trading that IBKR might not do on their own book, and you’ll have to wait for the T+2 settlement.
* Soon one day in some markets: Modernizing Trading Workflow: Getting Ready for T+1 | DTCC
** Though I believe if the buyer of the shares buys on the ex dividend date, they will also get the dividend, even if the shares are “in possession” of the settlement company for when the shares go ex dividend.