Edit: I research the internet to find some information about the matter since the first time I watched it. The first article I found was this:

More problematic than the believability of Matthew’s recovery, however, is another, apparently more minor, medical drama which plays out throughout the season. This involves Thomas Barrow (Rob James-Collier), the footman who, in Episode One, is shown serving as a Royal Army Medical Corps (RAMC) stretcher bearer. At the end of the episode, he gets a ‘Blighty’ wound by encouraging a sniper to shoot him through the hand. As Joanna Bourke has pointed out, self-mutilations, including those that involved courting danger as well asmore direct self-harm, were not uncommon as a way to malinger in wartime (1996: 83-7). However, while malingering could be viewed as an acceptable practice in certain contexts, ‘attempting to remove [one]self permanently from the war’ was viewed as a form of cowardice by military authorities and servicemen alike (McCartney 2005: 172). Thomas’s actions, building on his characterisation as duplicitous and conniving in Season One, appears to place his actions firmly in the negative category of cowardice. The association that is thus drawn between medical service and implicit cowardice is historically problematic, conflicting as it does with contemporary understandings of medicalservice personnel as ambiguous figures of heroism.

https://eprints.whiterose.ac.uk/135560/3/MeyerMathewslegsFINALcgJMedits.pdf

I will add the fact that this was done by the one character who is gay.