page-break-inline:avoid;
I think you mean
page-break-inside:avoid;
Some reader software and devices don't honor page-break-inside: avoid, although they mostly do ok with page-break-before:always and page-break-after:always. But try adding that to your .p-pb class, and see if it helps.
It does sound unusual, though, that your paragraph is splitting before the bottom border. I don't know if border-radius is allowed in epub --- what happens if you remove that from your css class?
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