Further info. This is happening on Chrome browser but not on Edge browser. I've cleared cookies and data with Chrome for the LastPass site. No change.
I closed my chrome tab for LastPass. I clicked the browser extension in Chrome and launched the "Vault" from there and opened a new tab. The new tab is now working fine/correctly and not overwriting/updating with the Master Password anymore. Not sure why this mattered but seems fixed.
The site password is overwritten by my Master password when I edit a site and click the eyeball (show password). Why?
I dont think the issue is to do with the browser password manager. The email address i use to login in to lastpass is also the same as i use on most sites. This is what seems to be causing the issue, if the email address of the site that im storing is the same as the lastpass email login then that is when the site password is overwritten with the lastpass password. If i change change the email address in the username field then the issue doesn't happen.... i hope that makes sense.