The behavior is related to the requirement for "rate conforming" in any NLE, which is magnified by the non-standard 80 fps frame rate. A timeline can only have one fixed frame rate. Clips placed on that timeline must either have the same frame rate, or something must be changed. For an 80 fps clip on a 60 fps timeline, there are three options:
(1) Match the rates frame-for-frame, which means slow-motion playback for the 80 fps clip.
(2) For normal-speed playback, some frames of the original clip must be discarded or duplicated, according to some algorithm. This is called "rate conforming." The default type of rate conforming in FCP is "Floor," which rounds down to the nearest whole frame number. It is efficient but does not produce the smoothest results. The rate-conforming method can be seen and adjusted in the FCP Video Inspector (CMD+4 toggles), under the heading "Rate Conform." This setting only appears if the clip frame rate differs from the timeline frame rate.
The need for rate-conforming predates digital video. When 24.0 fps movies are played on 29.97 fps TV, a type of rate conforming called 3:2 Pulldown (sometimes 2:3 Pulldown).
(3) Synthetically create new frames from image analysis of existing frames which can be smoothly blended to achieve the timeline frame rate while minimizing discarded/duplicate frames. This is normally called "optical flow rate conforming," however there are now more sophisticated methods involving machine learning. These settings are visible in the FCP Video Inspector as described above.
If using Optical Flow or Machine Learning, you click that to apply and just wait until the on-screen message says it is finished. Do not try to export it while that is running.
You may not see the above rate conforming artifacts in Premiere Pro because by default, it will alter the timeline frame rate of a new timeline to match the clip -- even if the clip is highly non-standard. That in turn produces a non-standard output, which if not well understood, will cause problems downstream. FCP does not do that but maintains standard frame rates and warns the editor with a pop-up that they are adding a non-standard clip to the timeline, and it will maintain a standard timeline frame rate.
