As Chris said, please attach files in the future.
After guessing what your values might be, I created the following file...
I adjusted the X placement of the second sweep so that you can see them side by side.
And here is a screenshot.
- Screenshot_20210422_094219.png (163.82 KiB) Viewed 653 times
The problem is not with the actual solid, it is with the visualisation of it. So in this case you could ignore the issue that you first posted about, however your last picture appears to be an invalid solid probably due to your not setting the transform of the section to Frenet, hence it self intersects as the circle attempts to be projected along the helix.
The more precise the visualisation, the more intensive the calculation task for your computer, hence the slower it gets. Where the best balance is for you will depend on your GPU, CPU, the nature of what you are attempting to model (its scale/ratio of the scale of its components, etc.) and personal preference.
The setting is under "PartDesign", here is a screen shot of the settings ...
- Screenshot_20210422_093943.png (56.1 KiB) Viewed 653 times
You can adjust those settings for better visualisations if you like but just be aware that if FreeCAD becomes rather slow, you may want to adjust them back again. Since the precision of the visualisation is based on the "bounding box" of the object, you are more likely to see issues when you have a small component working with larger components of the resulting solid, for example a small profile being swept along a long path is more likely to have a poor quality visualisation.
Jim
Who can help me with THAT, lol. Just kidding.