Your description is that the site fails to properly validate their input. This (weakly) implies a deep flaw in their code. If your input had simply choked their routine that calls
PBKDF2(), then your password hash might not have been reproducible; but I would expect a simple password reset should have been adequate to clear up that problem. Deleting your account might indicate that your account record was corrupt; however, deleting accounts might simply be their response to anyone who has a password problem due to unexpected user input. They might even be trying to actively thwart hackers with this response.
Also, flawed doesn't necessarily mean their site is vulnerable. The defective code would need to be exploitable, and you didn't supply evidence of that. Such evidence might include erratic behavior or inexplicably changed values.
If you decide to press further, perhaps testing individual password characters to isolate the glyph that caused their site to lock your account, know that they would be within their rights to consider those attempts to be a hacking attack. Seek the site owner's permission before experimenting.
Note that if instead of using high-bit-set characters, you construct your password from 16 cryptographically random, high-bit-unset, standard, ordinary, printable ASCII alphanumeric characters, the practical difference to your password's security will be irrelevant.