In T158684#2194099, @JerryK wrote:

Did you by any chance fire multiple shots? Zeds will still hear your silenced shots if you fire several shots in a quick succession.

Just found and used a SG5-K submachine gun and attached the suppressor to it. It works fine and it's quiet.
While I cannot say for sure, the but the issue seems limited to those 2 pistols.

Hello Geez and thank you for your time.
I just play simple survival/lone wolf, and sneak and headshot infected/zombies when looting houses and maybe trade and avoid other players.
So have not been actively trying to find other weapons.
I do have a double barrel shotgun but that does not take a Suppressor/silencer.
So the only 2 weapons affected that I know of are the 2 pistols mentioned,(.22 LR MK II & .380 IJ-70) it happens everytime and even attracts infected/zombies now when they did not before.
Yesterday I was in a big open field and there was one zombie and I shot him down with the .22 LR MK II and then more ran over from a far distance.
When this issue happens, the pistols are in pristine or worn condition, as well as the attached suppressor when using the .380 IJ-70.
They only other variable I can think of was a driver update from Nvidia at the time.
All I do know for sure is that this was not an issue during and before the Walpurgis Event.
Only after the event ended did this issue happen and it is constant at least for me.
I have even validated my files using Steam.