This is the thread @Peter_H was referring to (the offending character was a "):
The thread below also has a relevant discussion about the issue:
I believe that this is mainly an issue for various quotation marks (', ", `, ´, ‘,’,‛,“,”,‟) that may be auto-corrected into a different form preferred by the device you are typing on.
Another issue is that if traveling, you may find yourself using a keyboard that cannot produce the required characters (although some would argue you should never type in your master password on a device that isn’t yours, anyway).
With these caveats in mind, as long as one has access to at least one device on which it has been confirmed that a login can be successfully completed using a master password containing non-ASCII characters, it should be fine to stray from Bitwarden’s “standard” set of 71 characters. However, as noted by @JoshM, it would be prudent to back up the vault before attempting this.