So is there a possible workaround for monitors/KVMs that could cause this
issue? Sadly for 4-port DisplayPort KVMs with working 4K@60 there are very few
alternatives... most are either only DP 1.1 or only 4K@30.

Although I have to admit that this KVM (ATEN CS1924) is notoriously picky when
it comes to cables... only stock cables or some highly expensive DP1.4 cables
could make 4K@60 output work stable, but stock cables are only about 2m and in
reality I needed a few 3m cables (which cost me a handful to get the right
cables for the job) as some of the PCs are placed a bit far away from the KVM.