In the insert mode recording, markers from the previously recorded sound, do not move and are always locked to the timeline.
Correct. If you had, say, 150 markers and marker ranges already marked after you do an insert, that's an awful lot of markers you've got to update... Just the same thing happens if you insert material from another file into a file you've already marked - the markers stay in place on the timeline, because that's what they are - timeline markers, not content ones.
Yes, you could say that it's a bit of a limitation. But since it's fundamental to the way Audition works, the chances of it being altered are very slender, to say the least.
We have an option in preferences/markers and metadata for unlocking markers from timeline. but this option works only in edit mode, for example cut or paste.
This is true - but when you do that, it's a single computation, adding a known length of time to the file. The difference is that if you did this with a record insert, as it stands it would have to roll all the numbers. What you might like to do is to make a suggestion here asking for the option to update the timeline numbers when you've stopped recording the insert - because that would also be a single, fixed calculation and is clearly do-able.
Yes, this is do-able only when we do not use any marker during insert recording. Because new markers and old ones would be inseparable.