Shall I consider this behavior as a SW issue?
I should say so. You might even consider it a usability or accessibility issue. If something happens against what you expect you should file a bug. It can always be closed after investigation.
These 100 users can either log in to laptops, smartphones or tabs. How do I test this scenario?
Something like JMeter should do the trick nicely. @Dmitri T has lots of helpful links for a similar question over on StackOverflow.