That's always been the case on Alamy. I usually email after six months have elapsed since a sale was reported. It's not 'withholding payment'.

It's a feature which Alamy use because some buyers might prefer it. I believe it's standard on most macros, except they don't report the sale until it's paid for. For example, people find in-uses on Getty before the sale is reported. (This can also happen on Alamy e.g. via the UK newspaper scheme or distributor sales, which is why there's a 'Have you found any Alamy images?' thread each month on their forum.)

For future reference, on your My Alamy page, underneath the Overview tab near the top left of the page, you'll see your current status thus:
Current Cleared Balance: $XX   We'll pay you once your balance is over $75.

Is this the new business model? Photographers lending money to agency and clients?

Any thoughts on this?

As others have pointed out, it's not new. It is one way Alamy can justify higher prices there versus at other sites which don't give these generous credit terms. Long time between download and payment is an element of their business model.