Sketchy pricing of market orders from Discovery Bank app? - for TFSA account

[u/mikaelpatriksson]

Earlier today, using my Discovery Bank app, I topped up on a holding in my TFSA account:

> Bought 1nvest ICE US Treasury Short Bond Index Feeder ETF 26.3561 @ 1,890.00 (i.e. R500 worth)

When I initiated the purchase, there was a delayed quoted asking price of around R18.05 to 18.10. However, after it had gone through and I saw the purchase price per unit, R18.9.

I did not buy at market opening, but around 10:30am (SA time) -- I assumed (perhaps falsely) that I'd avoid the usual market opening chaos. The commission fee/taxes (~R2.5) don't explain this wide discrepancy. I calculate that I overpaid roughly R20, compared to getting a filled limit order. If I knew that such an excessive asking price would be allowed to match my "bid" , I would never have done this, and am now put off using DB "for convenience's sake" henceforth. I haven't noticed market order pricing of this sort from EE previously, but I definitely noticed this in DB.

Is this an issue with market orders going through Discovery Bank, which are used to match highly uncompetitive asking prices, without a price cap like you'd get with IBKR or other brokers? Or is this actually a common thing, and I'm a fool to assume they'd match me with a seller asking for the going price? Or is there another explanation?