Negative Lookbehind with \p{L} doesn't seem to work with ß and case insensitive flag

Why am I not getting a match when I have the case insensitive flag i here?

Regex Pattern:

\(?<!\p{L})x\i

Input:

ß x

Output: NO MATCH (WHY?)

Regex Pattern:

\(?<!\p{L})x\

Input:

ß x

Output: MATCHES x

Regex Pattern:

\(?<!\p{L})x\i

Input:

ß  x

(with two spaces)

Output: MATCHES x