I wanted to add Amecs to my modpack but I'm having this issue.
Pressing Ctrl or Alt makes it impossible to left click.
At the same time, if left click is pressed before pressing Ctrl or Alt, it makes left click being pressed nonstop.
A list of the mods I´m using can be found here
I don´t know how to find the culprit, but I don´t have this issue when not using Amecs.

I also have this issue.

It seems this happens when another mod has multi-key controls implemented. For example: inventory essentials has a few mappings to 3 simultaneous keys.
Unbinding these fixes the issue with Amecs but makes those specific key binds unusable or restricted.

Same here