A slight purple/violet tint on processed HP5+ is usually normal and is commonly due to residual film dyes, especially the anti-halation layer and other sensitizing dyes used in manufacture. These dyes are water-soluble and mostly wash out during processing, but some can linger after development.

It’s more related to washing/rinsing than to fixing time, though inadequate processing can leave more tint behind. In many cases the remaining dye will continue to fade with additional washing or over time.

For traditional black-and-white darkroom printing, this tint generally does not matter: B&W paper responds to light intensity, not the color of the film base. If you scan the negatives or print them with a color inkjet, the purple cast may show up unless you convert the image properly to grayscale/black-and-white.

So: slight violet film base = usually harmless residual dye, not necessarily a problem, and typically not visible in normal B&W prints.