Confirming with

Version: 6.0.5.2
Build ID: 54c8cbb85f300ac59db32fe8a675ff7683cd5a16
Threads CPU : 8; OS : Mac OS X 10.13.6; UI Render : par défaut; 
Locale : fr-FR (fr_FR.UTF-8); Calc: group

I'm pretty sure that this is a regression, but will need to check.

My tests were made with the native mysql connector extension (MacOS).

Reproduced also with
Version: 5.1.6.2
Build ID: 07ac168c60a517dba0f0d7bc7540f5afa45f0909
Threads CPU : 8; Version de l'OS :Mac OS X 10.13.6; UI Render : par défaut; 
Locale : fr-FR (fr_FR.UTF-8); Calc: group

Hmm, also reproduced with

Version: 4.1.6.2
Build ID: 40ff705089295be5be0aae9b15123f687c05b0a

maybe this bug has been around since the beginning of LO

Also reproduced with

Version 3.6.7.2 (Build ID: e183d5b)

OK, so am leaning more towards the "we've always had this problem" kind of bug...

Reproduced with OOo321m18 (build9502)

Setting as InheritedfromOOo

Tamás: noticing https://gerrit.libreoffice.org/#/c/55960/, thought you might be interested in this one.

Reproduced also with 

mysql-connector-java-5.1.36

so it isn't just limited to the native mysql connector.

Couldn't test it with native connector - isn't working here.

Tested with MariaDB and mariadb-java-client-2.2.3.jar. Created a new filed for a TIMESTAMP. Saved the table, closed the database-file, reopened the database-file: The TIMESTAMP hasn't been gone.

So I couldn't confirm this behavior for OpenSUSE 15, 64bit rpm Linux, MaraiaDB and Javaconnector for this MariaDB.

Hmm, inspection of the table definition created on the mysql server reveals that the timestamp field is created with the following attributes:

TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP

but the UI doesn't reflect this.

This has other side-effects with the jdbc-connector that cause the timestamp field to not even be displayed in the table data edit mode (double-click on table), even with the zeroDateTimeBehaviour=convertToNull flag added to the connection string.

On pc Debian x86-64 with master sources updated today + native mysqlc + 5.5.5-10.3.22-MariaDB-1 I could reproduce this.
I wonder if it could be due to the fact that in JDBC, there's only TIMESTAMP (see https://docs.oracle.com/cd/A97337_01/ias102_otn/buslog.102/bc4j/bc_abcdatatypes.htm) so not present in https://opengrok.libreoffice.org/xref/core/offapi/com/sun/star/sdbc/DataType.idl?r=19c4d8d2
Perhaps should we use "OTHER"?
79     /** indicates that the SQL type is database-specific and
80              gets mapped to an object that can be accessed via
81              the method com::sun::star::sdbc::XRow::getObject().
82      */
83     const long OTHER        = 1111;


DATETIME and TIMESTAMP are quite similar, according to Mysql website:
(https://dev.mysql.com/doc/refman/8.0/en/datetime.html)
 The DATETIME type is used for values that contain both date and time parts. MySQL retrieves and displays DATETIME values in 'YYYY-MM-DD hh:mm:ss' format. The supported range is '1000-01-01 00:00:00' to '9999-12-31 23:59:59'.

The TIMESTAMP data type is used for values that contain both date and time parts. TIMESTAMP has a range of '1970-01-01 00:00:01' UTC to '2038-01-19 03:14:07' UTC.

I began to give a try but stuck since I get "OBJECT" in the end.

Then when git grep the code, I noticed this:
offapi/type_reference/offapi.idl:10204:     ::com::sun::star::util::DateTime getTimestamp() raises (::com::sun::star::sdbc::SQLException);

So the 2 notions timestamp and datetime seem quite mixed.
=> uncc myself.

tested this again. The following behavior appears only with the direct connector of LO:
Open a table for editing, not for input date.
Create a new field as timestamp.
Save the table. The fieldtype won't be shown any more as timestamp, will be shown as datetime.
Field properties for the new field will be NOT NULL.
Open the same database with JDBC-connection. You could see the field is a timestamp field.

And now: Open the database in phpMyAdmin. The new timestamp-field has been created as NOT NULL DEFAULT CURRENT_TIMESTAMP() ON UPDATE CURRENT_TIMESTAMP().
It will be possible to create such a field for a table, which has already been filled with content in some rows. The timestamp will be automatically set to the timestamp this timestamp-field has been created.

So: There is no TIMESTAMP field modified to DATETIME. It is only shown as "DATETIME" in table editor with direct connection. Please test again.

(In reply to Robert Großkopf from comment #18)

Hi Robert,

> So: There is no TIMESTAMP field modified to DATETIME. It is only shown as
> "DATETIME" in table editor with direct connection. Please test again.

Yes, I think that is the real issue - the UI displays information that the field is defined as DATETIME whereas the underlying database field carries the correct definition.

So, it is still a UI bug at least.

Related questions would be whether or not that display value as "DATETIME" in the LO UI has other unintended consequences, e.g. how is data stored and/or entered when using the field in a form, for example, and what does it return in a query ?