I've done it a bunch of time in recent months with corporate TA-issued tickets (not Amex).

This was definitely the case in the past, the new flight options would either show $0 or some positive $ amount for price difference, there were no results with travel credits. Now I do see that some flights show a travel credit if the price is lower, although I haven't confirmed any of these options. What I think changed, which may or may not be related, is that our TA now has NDC capabilities and we can get the continuous pricing fares, not too long ago our corporate discount was more like a corporate penalty.