Missing plugin on import footage even though I have not yet started my project and I have not used any plugin

As unlikely as this is:

Select the DJI_0246.mp4 file in the Event browser and go to Clip > Open Clip. Then see if there's anything listed in the Effects Inspector on the right side above Compositing. It will just be like: one line with a label and a checkbox on the left if there is an effect applied that's missing. If so, you can select it and delete it.

If you accidentally applied a Default Video Effect (default shortcut Option-E) that was an Effects Preset with multiple effects, there may be more than one effect listed. The "missing ones" will just be that one line with no parameters I described above.

I downloaded your file, and imported it into FCP 10.6.8 without any issue.

We have been focusing so much on the codecs that we forgot to try one very basic trick in FCP users' toolbox... deleting preferences. We often say that when the application misbehaves (and it seems to be in this case), it is worth a try to delete the preferences... To delete preferences, hold down command and option while starting FCP X and click Delete Preferences. The application will start in a default state, and open an "Untitled" library.

Reopen the library you were working on, set the preferences the way you like them, and try again. Do the files still show the error?

I'm not sure QuickTime can play the iso2 version of AVC. Will it play in the QuickTime player? If not you could use Handbrake to convert it.

Please open that DJI_0246.mp4 file in MediaInfo and post a screenshot so we know more about the codec, etc.

It is visible in your screenshot that this file has .mp4 extension, whereas the other one has .mov, so not exactly the same thing.

Indeed, the two look identical, and it is a mystery why one would cause a "Missing Plug-in" warning.

Does that file play correctly in Quicktime Player? Does it help if you optimize the file in FCP or, if that fails, if you recompress the file using the Finder (control-click->Quick Actions->Encode Selected Video File), or Quicktime Player (File->Export)?

Would it be possible to upload one of clips to somewhere like Dropbox or WeTransfer so we could have a look at it?

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That's just the file information. We need the extended report from MediaInfo or Invisor.

I can’t say exactly how, but our collective experience over the years has shown that the preferences (plist) files for applications can often become corrupted-for example if the application crashes it will not have the opportunity to properly save its preferences, and then the file becomes corrupted. Many many many times as then FCP starts and reads the plist it then behaves oddly. How it can behave incorrectly in so many different ways when this happens is beyond my level of expertise.

What is the format of the media that you are importing?

Were they all recorded with exactly the same settings - codec, frame size, frame rate?