You should be in a 16:9 UI.
Does it play wide screen in media players like VLC?
If using traditional method not working, maybe treat the file as a PiP, and drag the corners to the proper width.
Using a 16:9 mask may help get to the correct aspect ratio. HP Envy Phoenix/4thGen i7-4770(4@3.4GHz~turbo>3.9)
Nvidia GTX 960(4GB)/16GB DDR3/
Canon Vixia HV30/HF-M40/HF-M41/HF-G20/Olympus E-PL5.
Tape capture using 6 VCR, TBC-1000, Elite BVP4+, Sony D8 camcorder with TBC.
https://www.facebook.com/BarryAFTT

Quote You should be in a 16:9 UI.
Does it play wide screen in media players like VLC?
If using traditional method not working, maybe treat the file as a PiP, and drag the corners to the proper width.
Using a 16:9 mask may help get to the correct aspect ratio.

In VLC it plays correctly if I tell it it its 16:9.

In PD I used PIP to unsquish it, which looks OK there, but once back on the timeline it is cropped

Please provide build of PD21.
Media info of original file.
Screenshot of timeline. HP Envy Phoenix/4thGen i7-4770(4@3.4GHz~turbo>3.9)
Nvidia GTX 960(4GB)/16GB DDR3/
Canon Vixia HV30/HF-M40/HF-M41/HF-G20/Olympus E-PL5.
Tape capture using 6 VCR, TBC-1000, Elite BVP4+, Sony D8 camcorder with TBC.
https://www.facebook.com/BarryAFTT

Quote Please provide build of PD21.
Media info of original file.
Screenshot of timeline.

Spent some time reproducing the problem, and now discovered the solution! Having changed the aspect ratio in PIP designer (changed width from 1 to 1.33), on returning to the timeline then need to change the width there as well to 16:9, otherwise it can crop (if too wide) or squish (if too narrow).