when using facter version >= 3.0, interfaces with uppercase characters are not saved. Root cause is a change in behavior between facter v2 and v3.

facter:

facterversion => 3.6.10
...
macaddress_Ethernet0 => 00:50:56:9b:4f:24
mtu_Ethernet0 => 1500
...
networking => {
  interfaces => {
    Ethernet0 => {
      mac => "00:50:56:9b:4f:25",
      mtu => 1500
    },
    eth0 => {
      mac => "00:50:56:9b:4f:24",
      mtu => 1500,
    },
...

foreman log (note the empty hash for ethernet0 facts):

...
2018-09-10T16:55:13 [D|app|ed4d9] We have following interfaces 'ethernet0, eth0' based on facts
2018-09-10T16:55:13 [D|app|ed4d9] Interface ethernet0 facts: {}
2018-09-10T16:55:13 [D|app|ed4d9] Interface eth0 facts: {"macaddress"=>"00:50:56:9b:4f:24", "mtu"=>1500 }
...