Galaxy S21, One UI 3.1, Android 11 has a feature that the screenshot's file name is suffixed with the current opened application name. The full file name format is: `Screenshot_{date}-{time}_${application_name}`. For example when taking a screenshot on Google chrome, the screenshot's file name will be "Screenshot_20210912-103624_Chrome.jpg" and when taking a screenshot when the Google Keep application is open, the filename will be: "Screenshot_20210912-103624_Keep Notes.jpg"
When I tried to add a screenshot file to one of my workout activities in Garmin Connect, it didn't present the screenshots with the sapces as one of the options. I thought it might be a file name length bug, or a galaxy bug - but the problem was only with spaces and all other apps didn't have any problem with it, so it does seems like a Garmin Connect bug.
I didn't find anywhere to reoprt this problem.