Swimming map

Maybe some activities were recorded as indoor swims or the GPS reception was really bad (even slightly under water there is no reception anymore).

If you are referring to your public activity, could be you haven't travelled outside of your privacy bubble if you have setting set to "Hide the start and end points of activities no matter where they happen"  (hides #.# mile/km of start)
...However on your own login feed... still should show privacy areas.

That can't be right, otherwise you could not use Strava for swimming?

What do you mean with "Use Strava for swimming"? Using the Strava App for recording a swim maybe? Do you know that a bunch of apps and devices exist that can record swims and uploads the recording to Strava? These aren't restricted to the Strava app shortcoming of only recording something with GPS data.

People rely on all kinds of convoluted tricks to record GPS on open-water swims, such as unnaturally throwing their hands over the water while swimming breaststroke; or keeping their watch inside their swim cap; just to allow it to connect to the satellites. You may have better luck with GPS while swimming backstroke, front crawl, and butterfly, but no guarantees.

That explains it