You need to provide more information about the test case and your environment. I cannot reproduce it using SQL Developer 19.4 on an 11g XE database for this table:
were the green "+" sign icon is available to create records
and the red "X" icon is available to delete any selected records
I get the same behavior if I create that table with the Wizard instead of the Worksheet. I get the same behavior if I create that table from the Other Users HR node while logged in as SYSTEM, and whether adding or deleting rows from the Object Browser logged in as SYSTEM or as HR.
So, what is your scenario and environment? Did it suddenly stop working? Did it work in a prior version? Etc, etc.
It is a very weird behaviour as this doesn't happen in other schemas when I PROXY CONNECT into them, however, when I PROXY CONNECT to this one schema, I'm noticing the buttons being greyed out.
I can create the table by running a DDL code from the worksheet and works fine. I can also perform a DML via code and it does COMMIT the record just fine. However, when I open that very same table with GUI, those buttons are greyed out and also, I cannot update or delete any records via GUI that I had previously inserted via code.
This behaviour doesn't happen in other schemas which I also PROXY CONNECT into them, so nothing special about those schemas compared to this particular one. Also, I don't think it's a bug in SQL Developer as it would have exhibited the same behaviour in ALL the schemas but that's not the case. Just trying to find out what is causing this table to be somehow "locked" via a GUI.
Yes, a weird one -- those make life interesting. We get to learn something, but often more than we want to know.
You might try examining the Statements - Log tab, specifically the SQL statements that get generated when you click on the Object Viewer's Data tab. For example, in the following screen shot, the SQL statements shown in Sequences 53 - 60 run when I click on the Data tab. The results of those probably determine which toolbar buttons SQL Developer enables. You can compare whether PROXY CONNECT to the schemas that work fine give a different set of statements, or whether running those individual statements yourself in the worksheet give significantly different results.
And, upon reflection, if you are using multiple copies of SQL Developer to access the different schemas, another possibility is that the Code Editor preference for the copy that is not working has been set to open the Object Viewer in read-only mode:
But if that is not the explanation, then it could be the one schema that does not work is in an older Oracle DB version that SQL Developer no longer supports. The lack of support could be intentional (many SQL statements for data dictionary queries are customized for different DB versions, but yours is no longer checked and instead gets a default query for some higher DB version), or unintentional (a newer JDBC driver version shipped with SQL Developer no longer works as well with an older DB version).
Perhaps other community members can think of other possibilities.
Thanks for sticking by Glen, I did check the Statements - Log tab but nothing different really when I compare between one schema that works and another that doesn't.
Also checked that 'Start in Read-Only mode' setting you referred to but it is not checked.