It is not the same sound. In Lexico, they have adopted a convention to replace the /ɜː/ with /əː/. It doesn't affect the pronunciation of the words containing this sound. If you look at A Key To English Pronunciations on their website, you're going to see it (her):
In Journal of the International Phonetic Association, Volume 33 Number 2 in December 2003, (http://www.yek.me.uk/ipadicts.html) you can read more about this change:
The third departure from EPD14b is the substitution of /ɜː/ with /əː/.
This has the advantage of reducing the total number of unfamiliar
symbols to be assimilated by the general user by an undeniably
If you look up other words with this sound, you will see it is just a matter of convention depending on the dictionary you are using. But the sounds /ɜː/ are not /əː/ pronounced the same.