An exit stamp in your passport is instant proof that you departed - as long as you are travelling on the same passport next time.

Sometimes Immigration Officers forget/don't bother to stamp passports. This is well known, and the lack of exit stamp in your passport will not be treated as automatic proof that you did NOT depart. It just means that you will have to prove your departure some other way - if they ask you (which they may not). If you get asked to prove your departure then an entry stamp from another country will certainly help.

But... it's much easier to simply ask for a stamp on departure!

And I didn't realise that they don't keep a record of passport scans. You learn something new every day.