In password security, the longer the better. With a password manager, using more than 24 characters is simple. Unless, of course, the secure password is not accepted due to its length. (In this case, through STOVE.)
Possibly indicating cleartext storage of a limited field (which is an absolute no-go), or suboptimal or lacking security practices.
Most likely it’s just a validation not related to actual storage of the information.
It’s something that can happen automagically when using a library. I wouldn’t be too surprised if this length limitation is just a default of whatever registration solution they are using.