According to digital ocean, their managed DB is a MySQL 8, so I don’t see why it wouldn’t work.
Additionally I couldn’t find on the web any reason why an ISO8601 string would not be inserted into a datetime column, in any version of MySQL.
I asked to see the server version to ensure that you’re not on a weird type of SQL that has different rules.
You can see the MySQL version in the WordPress admin dashboard (first item in the left-side menu). There should be a panel titled “Bitcoin and Altcoin Wallets”. It has several tabs, and there’s a “Debug” tab. Under it there’s some information. We’re interested in “MySQL version”.
Alternatively, you can type into your MySQL console this: SELECT VERSION(); This will give the same information.
As for the MySQL log, there should be a way for you to access it.
Since this is a strange issue, try to access the MySQL server log. It’s your best chance to figure out what’s wrong.
You can also use a temporary table to test and see if your server behaves as expected:
CREATE TEMPORARY TABLE timestamps( created_time DATETIME NOT NULL );
INSERT INTO timestamps VALUES('2020-08-06 10:00:00');
SELECT * FROM timestamps;
What do you get as result?