IMO that's the way it's supposed to work.
PSE (device which provides power) doesn't know how much power will PD (powered device) draw until after handshake finishes. So if PSE doesn't clearly have power budget to provide full capacity (802.3 at means 30W output from PoE port), it's logical (to me) that it doesn't even start the negotiation. Surely if PSE has multiple PoE out ports, it can do some power budget overbooking and in case power draw exceeds capacity, it disconnects power to select PDs (in order of priority). I don't think you would like to see that too often. With PoE in, device knows very well it's power budget: 25W less own consumption (up to 15W). With DC jack or 2-pin terminal device doesn't know power budget so it has to assume power available is ample enough for PoE out.
Next tricky thing might be PD negotiation ... PD should properly "signal" own power requirements ... however classification is optional and default is Class 0 with 15W power budget. Not sure if Mikrotik devices properly implement classicifation (I wouldn't be much surprised if they didn't). If PoE in power budget is 25W, own power consumption is 15W (it has to assume maximum consumption to avoid any instabilities), then remaining 10W is clearly not enough to power Class 0 PD with 15W power draw.