Actually the problem is different:

for Items the length constraints are working. For properties we skip the length constraints for label, description and aliases. I'm not sure whether that was by purpose, the code implies this..

byrial wrote:

The constraint for items seems to be lower than 250 characters which was indicated in bug:43278. Can you confirm that it is not 250 characters, but 250 bytes en UTF-8 encoding. And if so, is that by purpose or a bug?

Change 75115 had a related patch set uploaded by Tobias Gritschacher:
(bug 51813) apply length constraints to properties too

https://gerrit.wikimedia.org/r/75115

Change 75115 merged by Jeroen De Dauw:
(bug 51813) apply length constraints to properties too

https://gerrit.wikimedia.org/r/75115

Verified in Wikidata demo July 31th.