Hello @Avelon_N_A ,
Welcome to The MongoDB Community Forums! 
The behaviour you have described appears to match what is mentioned in the below blob from the $text documentation with regards to stemmed words:
Stemmed Words
For case insensitive and diacritic insensitive text searches, the
$textoperator matches on the complete stemmed word. So if a document field contains the wordblueberry, a search on the termbluewill not match. However,blueberryorblueberrieswill match.
If this is an Atlas deployment, you can take a look at Atlas search as it provides the autocomplete feature which performs a search for a word or phrase that contains a sequence of characters from an incomplete input string. You can also take a look at below blog which could help you in setting up partial search with Atlas search
In case you are interested in Atlas search and want to migrate your data from on-prem deployment to Atlas search you can follow
Additional resource to learn more about $regex, $text and Atlas search, please refer
Regards,
Tarun