Are you on Xbox, Steam or Microsoft Store version?
Microsoft Store
Are you using Developer Mode or made changes in it?
No
Brief description of the issue:
There are two big errors in the way the sim handles the DME element of ILS/DMEs.
1) The DME read incorrectly for approaches where it is supposed to be referenced to the runway threshold.
For example, London Heathrow 27R - the ILS DME is referenced to 0 NM at the threshold. The sim seems to be measuring from the runway mid-point and so the ILS DME over-reads by ~ 1 NM when comparing against GPS/VOR cross-cuts/distance-altitude checks (at 4 NM you expect to be at 1410 ft):
This issue seems to affect any airport where the ILS DME is meant to read zero at the threshold. The exact mis-match varies (I suspect based on runway length) at Newquay (EGHQ) it is ~ 0.7-0.8 NM out. It would seem like the sim is incorrectly referencing the runway mid-point.
The issue doesn’t seem to affect airports where there is only an ILS on one Runway, for example at Biggin Hill (EGKB) there is only an ILS to Runway 21 and in the sim the readings are within 0.1 NM of GPS and match altitude cross-checks (at 4 NM you expect to intercept the GS at 1800 ft):
Similarly, for approaches that don’t reference zero at the threshold (as is common in the USA) the sim seems to be providing a correct distance read-out. For example KJFK 4L:
It seems like the issue is at airports with an ILS to both runway ends (possibly where they share a frequency which is common in the UK) and the DME is referenced to zero at the threshold.
This is an issue because it significantly affects accurate IFR navigation and interactions with online ATC like VATSIM (you commonly get speed restrictions referenced to distance from touch-down based on an ILS DME which you cannot accurately comply with if the DME is incorrect.
2) For runways with an ILS to only one end and which have a suppressed back-course, the DME only reads in the 180 degree front-course sector.
I’ve detailed this before here:
This again is an issue primarily in Europe. There are airports with an instrument approach to the non-ILS runway that require the DME element of the ILS, ie. an NDB/DME approach to the opposite runway end. You also commonly require the DME for any missed approach procedures for the ILS approach. These are impossible to fly if you use any add-on scenery or navigation data that suppresses the back-course (correctly at the affected airports). Again, this significant affect IFR navigation and can make it impossible to fly certain IFR procedures.
Both of these are issues with the basic mechanics of how an ILS DME station works and both significant affect the experience for people trying to accurately emulate IFR procedures. I would be very grateful if they could be registered and fixed! 
Thanks!
PC specs and/or peripheral set up if relevant:
N/A
Build Version # when you first started experiencing this issue:
Noted in multiple version - I suspect present since initial release
