I've dealt with Enerprof.de and found them nothing less than professional.the chargers came with the bundle with the battery, you can find them here, i have 5 amp and 8 amp versions with 54 volt current and xlr-3 connections https://enerprof.de/en/collections/chargers-lifepo4-15s-54v and im not going to stick my multimeter to to the xl3-r outputs because i dont know what holes are positive and negative and what is the "ground" wire, its enough evidence to me that i tried both chargers to see if there is any difference in the battery max voltage after charging and there is no difference, so i dont believe the chargers are defective.
With those charger plugs, you can unscrew the plastic collar and the plug comes apart to see which is plus and minus and measure more easily.
Your expectation to "see around 54.75V" is unrealistic given the charger output is capped at 54v, so maximum charge will be somewhat less than 54v.
Enerprof also do higher voltage chargers up to 58v for their 52v packs.