The value for tx power is reported incorrectly

The value for tx power is reported incorrectly:

root@MX8500:/usr/bin# ubus call iwinfo txpowerlist '{"device": "phy5-ap0"}'
{
	"results": [
		{
			"dbm": 255,
			"mw": 2147483647,
			"active": false
		}
	]
}

iwinfo displays this value correctly after the last update:

root@MX8500:/usr/bin# iwinfo phy5-ap0 txpowerlist
   0 dbm (   1 mW)
   1 dbm (   1 mW)
   2 dbm (   1 mW)
   3 dbm (   1 mW)
   4 dbm (   2 mW)
   5 dbm (   3 mW)
   6 dbm (   3 mW)
   7 dbm (   5 mW)
   8 dbm (   6 mW)
   9 dbm (   7 mW)
  10 dbm (  10 mW)
  11 dbm (  12 mW)
  12 dbm (  15 mW)
  13 dbm (  19 mW)
  14 dbm (  25 mW)
  15 dbm (  31 mW)
  16 dbm (  39 mW)
  17 dbm (  50 mW)
  18 dbm (  63 mW)
  19 dbm (  79 mW)
* 20 dbm ( 100 mW)
  21 dbm ( 125 mW)
  22 dbm ( 158 mW)
  23 dbm ( 199 mW)
  24 dbm ( 251 mW)

Reproduce the issue with ubus call iwinfo txpowerlist for phy5-ap0 and compare it with iwinfo phy5-ap0 txpowerlist. Trace the txpowerlist RPC entry point in rpcd and identify why the JSON reports 255 dBm and 2147483647 mW instead of the values shown by iwinfo. Done means the ubus response reports the correct tx-power list.