Using Export Frame on any clip with a proxy attached (even if proxies not active) results in JPGs and PNGs in the proxy-resolution. This seems to happen at random.
During some troubleshooting, oddly it seemed to matter which window (ie Program, Timeline, etc..) was active (highlighted in blue) but this actually reversed between different projects, and ultimately using Export Frame results in low-res and full-res frame grabs at random.
Clips that don't have proxies don't seem to have this issue.
Native media in my examples were .mxf and .mp4
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