I’m beginning to think it’s a bug. There should be a simple recipe to convert your existing (and pricy) 5AR /5 something infusions into +7AR / +5 something infusions.
Can’t just leave people in the competitive darkness when it comes to legacy 5/5 infusions. See also https://forum-en.gw2archive.eu/forum/game/dungeons/fractal-infusion-request-7-AR-5-someth/first#post5818279
icy`