To answer the question in the topic: It shouldn't. The whole "Lionsteel's M390 is too soft" debate started with a youtube video from Luvthemknives. LTK stated that a Collector's knives sheep foot made by Lionsteel only had a 51 HRC hardness despite being advertised as 58-60 HRC. In the end it turned out that it was indeed 59.1 HRC and LTK deleted his video. In his follow up video (also deleted) LTK admitted that not a single knife was out of specification.
I recommend reading the original thread here on blade forums and Collectorknives' comment under this Nick Shabazz video:
My personal opinion: The whole thing really went out of proportion and some people that do not like Lionsteel for winning too many blade awards saw it as an opportunity to post bs.
To sum things up: Lionsteel knives usually have the hardness that is advertised. Is 58-60 HRC too soft? That is open to debate. Several European knife companies regard it as the optimal range for an edc or outdoor knife. However, there are also people like their M390 hardened to 62 HRC - that's what Spyderco did with some Para 2 exclusives (AFAIK the Para 3 M390 exclusive was also tested at 59 HRC though).
I have owned at least 30 knives with M390 blades and I think Lionsteel usually does a pretty good job hardening the blade to the sweet spot where it keeps a good edge while not becoming brittle or even prone to clipping.