Strictly speaking, the ether2-8 should only accept Passive PoE in, (as mode B), so unless there is a setting for Passive PoE out on the switch (AND Mode B) , the RB5009 should not power on from any of them if 802.3af/at/bt is provided.
Roughly, there is usually a pre-detection of the connected device on the power supply side:
- a resistance below 3 KOhm or so means that the device is NOT PoE compatible and power should NOT be applied to that port
- a resistance above 3 KOhm or so means that the device might be Passive PoE compatible so power may be applied to the port (if settings are correct)
- a resistance above 26 KOhm or so means that the device might be 802.3af/at/bt compatible so power may be applied to the port (if settings are correct AND power negotiation is carried successfully)
So what you report is strange.
Many devices (but not Mikrotik) use Mode A for passive, in the 802.3af/at/bt scheme, the one "in command" is (should be) the PSE (the switch in this case) that "tells" the PD (the RB5009 in this case) that it is going to supply power either in Mode A or Mode B and the PD should comply with that.
I am not at all familiar with the settings available on that USW-Flex-2.5G-8-PoE device, maybe you got it working "forcing" passive PoE out and the switch actually supports Mode B.
But if the negotiation is not carried properly (no matter if the fault is on the switch or on the router side) there won't be voltage out, this is by design in 802.3af/at/bt.
What is happening (maybe) is that the switch initiates the connection but this does not go on or doesn't complete in the allowed timeframe:
https://en.wikipedia.org/wiki/Power_over_Ethernet#Standard_implementation
The correspondence is:
PoE = 802.3af
PoE += 802.3at
PoE ++= 802.3bt
But negotiation should start "from the bottom", i.e. first test for af, then for at and finally for bt, so your switch that provides PoE ++, i.e. 802.3bt should be -in theory - compatible withe the router, that should accept 802.3af/at (this is clearly a mistake in the specs of the RB5009, a device that "idles" at 14 W cannot be powered by 802.3af that maxes out at 12.95 W ) 802.3at should be needed.
If you have mains power near the switch and not near the RB5009, probably your only way out is to use an external 802.3at power injector, or a 802.3at splitter near the router but 2.5 Gb versions of these devices are usually industrial (please read as "not cheap").
If you need to use PoE OUT from the RB5009 the use of the 802.3at power injector is ruled out, see:
RB5009 / PoE-IN *and* PoE-OUT at the same time
(the RB5009UPr+S+IN can seemingly provide PoE out only if powered form DC jack or 2 pin connector)
BUT, if you power the RB5009 through PoE (802.3at splitter to either power jack or 2 pin connector) you would only have 25.50 W - 15 W = 10.50 W available for PoE out from the RB5009.
So you would need an 802.3bt 2.5Gb splitter and that would be even more costly.