saved properties of classic graphs in v6.0 are lost when open graph again

It seems that some UI was broken in version 6.0 (affected also 6.0.1 and 6.0.2).
It's impossible to save modified values of some attributes for the classic graphs.

Steps to reproduce:

1. Navigate to "Configuration" -> "Hosts" from the main menu of Zabbix web interface;
2. Click on the "Graphs" link for any host containing a classic graphs (non-inherited from the template);
3. Click on any graph with "Graph type"="Normal" (or just create a new graph, save and then open it for editing);
4. Change attributes "Draw style" and "Y axis side", possible - some other fields, click "Update" button, then open this graph for editing again.

Result:
You see the old values in fields "Draw style" and "Y axis side".
All other fields contain their new (modified) values.

Expected:
All attributes (including the "Draw style" and "Y axis side") should be saved.

Zabbix v5.0.x did not have this bug.