Transparency issues with Refine Tool in Select and Mask

Hi community,

Currently having issues trying to separate the folowing model from the background, specifically in regards to hair. From all the tutorials and videos I've seen, the process of making a selection, going to Select And Mask and then using the Refine Edge tool around hair seems simple enough. For some odd reason, when I use the Refine Edge tool, the results bear horrible transperancy issues. It's as if the tool is struggling to find the subject. See below (in this order: 1, 3 and 2):

If I select around said subject and then try to use refine tool to find her hair, I get the same weird transparency issues. Is this a settings thing? Or am I completely missing part of the process? Thanks!

I currently work with a MacBook Pro (16-Inch, 2019)

Processor: 2.6 GHz 6-Core Intel Core i7

Memory: 16 GB 2667 MHz DDR4

Best,

Uba