MariaDB Parallel Replication Drifting at random times
innodb_file_per_table = On  -- only applies during CREATE TABLE or ALTER TABLE
table_open_cache = 8192
table_cache = 70000  -- old name for table_open_cache; 70K is too high
init-connect='SET NAMES utf8'  -- keep in mind that user=root skips init_connect
long-query-time = 30  -- so high as to be virtually useless


"one of these only has a 2.6G ibdata1 file" -- What do you mean?

"But some servers will just not correct" -- Do you mean 'some Slaves'?  Or 'some Clients'?

"no latency ~6ms ping and stable" -- that implies several hundred miles or km.

"Some servers drift" -- clocks drift; servers don't.  What do you mean?

"3 hours behind" -- Do SHOW SLAVE STATUS;

Replication existed for more than a decade before GTID was added.  That won't be to blame for what you are seeing.