Replace Specific Frames and Interpolate Them?

[PizzaTime]

Unfortunately, it does not appear to be doing anything.

Okay. You did use the Vapoursynth preview to check that before encoding, right? (at least to make sure your settings are correct)

Yes. Nothing was changing in the preview but I tried encoding anyway just to see if preview was giving an error. As I have never used the filter before. Debug within the preview window was saying that those frames I chose were "Input(before scene change)".

I'm going from Huffyuv>deinterlace to UT Video (for editing in other programs)>Encode to UT Video again, but with Frame Replace Filter

According to the debug output, no deinterlacing is done, since the source is detected as progressive. This will disable the deinterlacing.
If your source is progressive and not properly flagged, then you should let Hybrid know that by setting 'Filtering->(De-)Uninterlace/Telecine->Overwite input scan type' to the correct value. Note that if a source is loaded and detected as interlaced, the 'Auto deinterlace handling' setting isn't there,....

Sorry. Yes, this version is progressive. The video I am trying to use this filter with has already been deinterlaced previously with Hybrid. I was just mentioning my workflow in case that was relevant to the ticket/troubleshooting. Sorry if I caused any confusion.  

As the source was too big to attach (even for 5-7 seconds).

Then upload a portion of your source somewhere i.e. GoogleDrive or similar and send me a link via PM, in case it's privacy relevant.
Cu Selur

Will do.