Something in the RO/RSS/RP-1 Legacy Modpack is preventing parts from being placed, unless behind a menu

[Geostationary Syncom]

I am currently using the RP-1 Legacy modpack that TheBeardyPedguin provided as its the only place I could find a Legacy Modpack, though I did not download many of the extra part mods, as I did not want them, however, this is a bug I have been having since around 2.0 came out. Perhaps before but I cant remember. Whenever I grab a new part, it will sometimes freeze and be unmovable from where it spawned in, but if i drag my mouse behind a menu, such as the MechJeb deltav readout, the part can be moved and placed, but not picked back up again, leaving the VAB does not fix it, switching save files does not fix it, only resetting the game fixes it, this makes the game basically unplayable, needing to restart it every few parts. I have looked and have not found any mention of this bug and am desperate for a fix. Does anyone know what causes this or how to fix it? The debugging menu doesnt say anything when it happens, and it's incredibly confusing.