Appears your mod (after much testing, and narrowing down: I troubleshooted by loading up a world ready to load a rocket with various selections of mods to test for end game dialogue and achievement processing: OIC prevents both, somehow. I understand this could be by design as your mod only really provides content for after rocket launches, but due to the way many of the achievements function, and the "end of game" being determined by the launch of a rocket, and any play afterwards shouldn't necessarily count "against" those achievements, i feel a different method should be looked for. Unless, there is some other way to trigger "end game" with this mod installed that still enables the calculation of those achievements, players wishing to get any of those cannot do so with this mod (or FT tweaks was another culprit as well during my testing) installed.

Interesting; in testing, the "Game finished" dialogue still appeared for me, but I don't test for achievements, so I can't comment on that.

I do see that the core game silo-script.lua seems to have been updated to v2 at some point, and I haven't updated the mod's interface with it since v1. However, it does not seem this would cause any problems unless you have an old savegame that was created in an old Factorio version that was running v1 of the silo-script (or from before silo-script was split to its own file). Were you testing using a savegame originally created in, say, Factorio 0.15.X? If so, then I can understand why things are not functioning as intended.

Do note that achievements earned while mods are active are counted separately from vanilla achievements anyway, and I believe they won't be synced with the list of completed Steam achievements.

that may have been part of the issue, or may have been that i originally created the world with no mods installed, except for creative mode mod (instant research and instant items) and a few other time saving things just to get me to a functional silo i could ready-launch. without creating the world with OIC installed originally, could that have impacted it?

I'll try a few more test saves, this time creating the map with OIC installed initially

yup,i tried again, this time with OIC installed (and FtTweaks not) when the world was built, and now i'm trying to remember under what conditions i originally ran the test, my notepad++ info is gone where i had jotted down my notes from different game states (and what mods where loaded and any interactions it may have caused). Is it possible, for, if, say, another mod is installed and then removed, for a change it made to the silo script to not be reversed? (like if FTweaks changed something in the save file, and then after removing the mod, the change wasn't reverted??) that would definitely have screwed with my findings.

at any rate: your mod doesn't seem to have been the cause, and i think it likely now, that even though i loaded up the same save file multiple times and it only failed to issue the end-state if either (or both) ftweaks /and/or OIC were installed to have meant originally that they both were responsible in some way.