I spoke to the developer and he asked if we could see how and where you are entering it in the back end in the locations panel? As mentioned above I noticed you have another plugin called Google My Maps, so I have no idea if that is effecting the redirect, but when I brought this up at our meeting he said it will convert images to the same as sites directory path url (http toor https) where images are stored otherwise you will get error messages in your console about not loading resources but we never heard of this happening with SLP website links . Are you using Network Active Multisites? Something else is converting the site links, perhaps something in your theme or in your map plugin.
What does your SLP Results Layout look like for the attribute website links? For instance Mine is “slp_result_contact slp_result_website”>[slp_location web_link] , have you modified the SLP Plugin style using CSS by any chance?
I could not reproduce your situation. The steps I took to try and reproduce
1) . I added a brand new location randomly picked off the internet, that had an http not https. I added it and copied the link to PO Melvins exactly as it appeared in my browser.
2) updated and went back to my Locations and found PO Melvins, I hovered over the link.
I have attached what it looks like in the back end, and what it looks like on the front end.