Something does seem amiss, or we are missing something. It appears that either iMovie isn't functioning correctly or your clip is not 1080. Double check the resolution by opening the clip in QuickTimePlayer and doing a Window/Show Movie Inspector.
If the resolution checks out as 1080 then you might consider calling Apple Support and see if they can help. To do that click on the Contact Support item in the upper right hand corner of this forum's screen and navigate to a phone number. Apple Support has the capability of remotely accessing your computer, that we cannot do here.
You might also consider updating your iMovie and upgrading your High Sierra to current versions. Catalina is the current OS version. However, Catalina won't support 32 bit applications, so if you rely on any older 32 bit applications you might not want to do the upgrade yet. Also, during its breaking in period Catalina has exhibited some quirks that have not yet been fully worked out. The new Photos 5 in particular, that is built in to Catalina, has had a lot of issues.
-- Rich