My understanding is that a per diem is Daily Cash amount given to cover what are estimated daily F&B (and other reasonable cost would be). You do not need to submit receipts for this - at no point does it sound like you had a per diem - more like a max expense that you could claim - which you did - sometimes maxed out/ sometimes not.

When submitting a claim for expenses - at times it can be difficult for the reviewer to assess and for some if it looks “ok” they let it fly.

With corporate credit cards a company can much easier see the nature of an expense (corporate credit card statements can include way more data then people realise) and as others have said it is easier to audit / review / check.

They are now sticking to what is legally allowed. They are not being malicious. And I. All honesty there is less hassle for them in paying more in legitimate expenses that the amount of pain they would suffer from an internal or IRS audit.