I should add for clarity and completeness: HBSC to Monzo bank transfer (via Faster Payments) that don’t happen instantly are just a minority of cases. Transfers that get credited the following day are even less. HSBC does advise customers of the possibility of delays to outgoing Faster Payments in certain circumstances.
As @anon44204028 mentioned earlier in the thread, this is all within FPS timeframes.
If you are concerned of delays in incoming bank transfers to your Monzo account from another bank, get in touch with support via the in-app chat, ideally quoting: amount expected, sort code, account number, and name of the sender, and date/time the transfer was made. We may be able to help straight away, or we’ll refer your case to someone who has direct access to the FPS network and can check more details